fix unlikely corner cases in getopt's message printing

like fputs (see commit 10a17dfbad2c267d885817abc9c7589fc7ff630b), the
message printing code for getopt assumed that fwrite only returns 0 on
failure, but it can also happen on success if the total length to be
written is zero. programs with zero-length argv[0] were affected.

commit 500c6886c654fd45e4926990fee2c61d816be197 introduced this
problem in getopt by fixing the fwrite behavior to conform to the
requirements of ISO C. previously the wrong expectations of the getopt
code were met by the fwrite implementation.

add error message printing to getopt_long and make related improvements

some related changes are also made to getopt, and the return value of
getopt_long in the case of missing arguments is fixed.

support translation for getopt error messages

fix stderr locking and ferror semantics in getopt message printing

if writing the error message fails, POSIX requires that ferror(stderr)
be set. and as a function that operates on a stdio stream, getopt is
required to lock the stream it uses, stderr.

fwrite calls are used instead of fprintf since there is a demand from
some users not to pull in heavy stdio machinery via getopt. this
mimics the original code using write.

fix getopt handling of initial '+' in optstring

in the case where an initial '+' was passed in optstring (a
getopt_long feature to suppress argv permutation), getopt would fail
to see a possible subsequent ':', resulting in incorrect handling of
missing arguments.

fix getopt handling of ':' modifier for multibyte option characters

the previous hard-coded offsets of +1 and +2 contained a hidden
assumption that the option character matched was single-byte, despite
this implementation of getopt attempting to support multibyte option
characters. this patch reworks the matching logic to leave the final
index pointing just past the matched character so that fixed offsets
can be used to check for ':'.

add support for non-option arguments extension to getopt

this is a GNU extension, activated by including '-' as the first
character of the options string, whereby non-option arguments are
processed as if they were arguments to an option character '\1' rather
than ending option processing.

getopt: fix optional argument processing

Processing an option character with optional argument fails if the
option is last on the command line. This happens because the
if (optind >= argc) check runs first before testing for optional
argument.

support optional-argument extension to getopt via double-colon

this extension is not incompatible with the standard behavior of the
function, not expensive, and avoids requiring a replacement getopt
with full GNU extensions for a few important apps including busybox's
sed with the -i option.

Add ABI compatability aliases.

GNU used several extensions that were incompatible with C99 and POSIX,
so they used alternate names for the standard functions.

The result is that we need these to run standards-conformant programs
that were linked with glibc.

add getopt reset support

based on proposed patches by Daniel Cegiełka, with minor changes:
- use a weak symbol for optreset so it doesn't clash with namespace
- also reset optpos (position in multi-option arg like -lR)
- also make getopt_long support reset

initial check-in, version 0.5.0