byte-based C locale, phase 2: stdio and iconv (multibyte callers)

this patch adjusts libc components which use the multibyte functions
internally, and which depend on them operating in a particular
encoding, to make the appropriate locale changes before calling them
and restore the calling thread's locale afterwards. activating the
byte-based C locale without these changes would cause regressions in
stdio and iconv.

in the case of iconv, the current implementation was simply using the
multibyte functions as UTF-8 conversions. setting a multibyte UTF-8
locale for the duration of the iconv operation allows the code to
continue working.

in the case of stdio, POSIX requires that FILE streams have an
encoding rule bound at the time of setting wide orientation. as long
as all locales, including the C locale, used the same encoding,
treating high bytes as UTF-8, there was no need to store an encoding
rule as part of the stream's state.

a new locale field in the FILE structure points to the locale that
should be made active during fgetwc/fputwc/ungetwc on the stream. it
cannot point to the locale active at the time the stream becomes
oriented, because this locale could be mutable (the global locale) or
could be destroyed (locale_t objects produced by newlocale) before the
stream is closed. instead, a pointer to the static C or C.UTF-8 locale
object added in commit commit aeeac9ca5490d7d90fe061ab72da446c01ddf746
is used. this is valid since categories other than LC_CTYPE will not
affect these functions.

remove outdated and misleading comment in iconv.c

the comment claimed that EUC/GBK/Big5 are not implemented, which has
been incorrect since commit 19b4a0a20efc6b9df98b6a43536ecdd628ba4643.

in iconv_open, accept "CHAR" and "" as aliases for "UTF-8"

while not a requirement, it's common convention in other iconv
implementations to accept "CHAR" as an alias for nl_langinfo(CODESET),
meaning the encoding used for char[] strings in the current locale,
and also "" as an alternate form. supporting this is not costly and
improves compatibility.

add hkscs/big5-2003/eten extensions to iconv big5

with these changes, the character set implemented as "big5" in musl is
a pure superset of cp950, the canonical "big5", and agrees with the
normative parts of Unicode. this means it has minor differences from
both hkscs and big5-2003:

- the range A2CC-A2CE maps to CJK ideographs rather than numerals,
contrary to changes made in big5-2003.

- C6CD maps to a CJK ideograph rather than its corresponding Kangxi
radical character, contrary to changes made in hkscs.

- F9FE maps to U+2593 rather than U+FFED.

of these differences, none but the last are visually distinct, and the
last is a character used purely for text-based graphics, not to convey
linguistic content.

should there be future demand for strict conformance to big5-2003 or
hkscs mappings, the present charset aliases can be replaced with
distinct variants.

reportedly there are other non-standard big5 extensions in common use
in Taiwan and perhaps elsewhere, which could also be added as layers
on top of the existing big5 support.

there may be additional characters which should be added to the hkscs
table: the whatwg standard for big5 defines what appears to be a
superset of hkscs.

add Big5 charset support to iconv

at this point, it is just the common base charset equivalent to
Windows CP 950, with no further extensions. HKSCS and possibly other
supersets will be added later. other aliases may need to be added too.

iconv support for legacy Korean encodings

like for other character sets, stateful iso-2022 form is not supported
yet but everything else should work. all charset aliases are treated
the same, as Windows codepage 949, because reportedly the EUC-KR
charset name is in widespread (mis?)usage in email and on the web for
data which actually uses the extended characters outside the standard
93x94 grid. this could easily be changed if desired.

the principle of this converter for handling the giant bulk of rare
Hangul syllables outside of the standard KS X 1001 93x94 grid is the
same as the GB18030 converter's treatment of non-explicitly-coded
Unicode codepoints: sequences in the extension range are mapped to an
integer index N, and the converter explicitly computes the Nth Hangul
syllable not explicitly encoded in the character map. empirically,
this requires at most 7 passes over the grid. this approach reduces
the table size required for Korean legacy encodings from roughly 44k
to 17k and should have minimal performance impact on real-world text
conversions since the "slow" characters are rare. where it does have
impact, the cost is merely a large constant time factor.

fix iconv conversion to legacy 8bit codepages

this seems to have been a simple copy-and-paste error from the code
for converting from legacy codepages.

use restrict everywhere it's required by c99 and/or posix 2008

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

fix multiple iconv bugs reading utf-16/32 and wchar_t

fix iconv dest utf-16: unavailable chars must be replaced; EILSEQ is wrong

fix erroneous utf-16 encoding with surrogates in iconv

apparently this was never tested before.

fix major breakage in iconv, bogus rejecting of dest charsets

gb18030 support in iconv (only from, not to)

also support (and restrict to subsets) older chinese sets, and
explicitly refuse to convert to cjk (since there's no code for it yet)

legacy japanese charset support in iconv (only from, not to)

simplify iconv and support more legacy codepages

iconv was not returning -1 on most failure

this broke most uses of iconv in real-world programs, especially
glib's iconv wrappers.

fix breakage due to converting a return type to size_t in iconv...

fix all implicit conversion between signed/unsigned pointers

sadly the C language does not specify any such implicit conversion, so
this is not a matter of just fixing warnings (as gcc treats it) but
actual errors. i would like to revisit a number of these changes and
possibly revise the types used to reduce the number of casts required.

use a more-correct integer type, and silence 64-bit warnings as a bonus

initial check-in, version 0.5.0