MFC r320472,r320508,r320509:
Make stdio deferred cancel-safe.

Requested by: eugen

MFC r305406,r305409,r305412

1) Fix errors handling.

2) Prevent out of bounds access to ws[-1] (passed buffer) which happens
when the first mb sequence is incomplete and there are not enougn chars in
the read buffer. ws[-1] may lead to memory faults or false results, in
case the memory here contains '\n'.

3) Fix n == 1 case. Here should be no physical read (fill buffer) attempt
(we read n - 1 chars with the room for NUL, see fgets()),
and no NULL return.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Implement xlocale APIs from Darwin, mainly for use by libc++. This adds a
load of _l suffixed versions of various standard library functions that use
the global locale, making them take an explicit locale parameter. Also
adds support for per-thread locales. This work was funded by the FreeBSD
Foundation.

Please test any code you have that uses the C standard locale functions!

Reviewed by: das (gdtoa changes)
Approved by: dim (mentor)

Eliminate more dead stores.

Found by: Clang static analyzer
MFC after: 7 days

Next stage of stdio cleanup: Retire __sFILEX and merge the fields back into
__sFILE. This was supposed to be done in 6.0. Some notes:
- Where possible I restored the various lines to their pre-__sFILEX state.
- Retire INITEXTRA() and just initialize the wchar bits (orientation and
mbstate) explicitly instead. The various places that used INITEXTRA
didn't need the locking fields or _up initialized. (Some places needed
_up to exist and not be off the end of a NULL or garbage pointer, but
they didn't require it to be initialized to a specific value.)
- For now, stdio.h "knows" that pthread_t is a 'struct pthread *' to
avoid namespace pollution of including all the pthread types in stdio.h.
Once we remove all the inlines and make __sFILE private it can go back
to using pthread_t, etc.
- This does not remove any of the inlines currently and does not change
any of the public ABI of 'FILE'.

MFC after: 1 month
Reviewed by: peter

Don't add integers to void pointers.

Read directly from the stdio buffer using the new __mbsnrtowcs() interface
instead of making repeated calls to __fgetwc().

Lock the file once per call and use the unlocked fgetwc()/fputwc() variants.

Reimplement the functionality of fgetrune(), fputrune(), and fungetrune()
here in terms of mbrtowc(), wcrtomb(), and the single-byte I/O functions.
The rune I/O functions are about to become deprecated in favour of the
ones provided by ISO C90 Amd. 1 and C99.

Style: One space between "restrict" qualifier and "*".

Basic support for wide character I/O: getwc(), fgetwc(), getwchar(),
putwc(), fputwc(), putwchar(), ungetwc(), fwide().