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

Requested by: eugen

MFC r305241

fgetwc(3) may set both __SEOF and __SERR at once (in case of incomplete
sequence near EOF), so we can't just check for
(wc == WEOF && !__sfeof(fp)) and must relay on __sferror(fp) with
__SERR clearing/restoring.

MFC r304607,r304641,r304819,r304811

1) Don't forget to set __SERR on __slbexpand() error.

2) Remove "Fast path" from fgetwc()/fputwc() since it can't detect
encoding errors and ignores them all.
One of affected encoding example: US-ASCII

3) Original fgetln() from 44lite return success for line tail errors,
i.e. partial line, but set __SERR and errno in the same time, which
is inconsistent.
Now both OpenBSD and NetBSD return failure, i.e. no line and set error
indicators for such case, so make our fgetln() and fgetwln()
(as its wide version) compatible with the rest of *BSD.

PR: 212033

MFC r288006,r288031,r288032,r288033:

r288006 (by rodrigc):

Add declarations to eliminate -Wmissing-prototypes warnings

r288031 (by rodrigc):

Remove names from some prototypes

r288032 (by rodrigc):

Remove names from some prototypes

r288033 (by rodrigc):

Use ANSI C prototypes. Eliminates -Wold-style-definition warnings.

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)

Fix an off-by-one bug that caused the first character of the buffer to
be uninitialized.

Add fgetwln(), a wide character version of fgetln().