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

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

If the size passed to {,v}s{w,n}printf is larger than INT_MAX+1
(i.e., the return value would overflow), set errno to EOVERFLOW
and return an error. This improves the chances that buggy
applications -- for instance, ones that pass in a negative integer
as the size due to a bogus calculation -- will fail in safe ways.
Returning an error in these situations is specified by POSIX, but
POSIX appears to have an off-by-one error that isn't duplicated in
this change.

Previously, some of these functions would silently cap the size at
INT_MAX+1, and others would exit with an error after writing more
than INT_MAX characters.

PR: 39256
MFC after: 2 weeks

Ensure that the {,v}swprintf functions always null-terminate the
output string, even if an encoding error or malloc failure occurs.

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)

- Use an initializer macro to initialize fields in 'fake' FILE objects used
by *sprintf(), etc.
- Explicitly initialize _fl_mutex to PTHREAD_MUTEX_INITIALIZER for all FILE
objects. This is currently a nop on FreeBSD, but is import for other
platforms (or in the future) where PTHREAD_MUTEX_INITIALIZER is not simply
zero.

PR: threads/141198
Reported by: Jeremy Huddleston @ Apple
MFC after: 2 weeks

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

Fix EOVERFLOW detection in vswprintf(3)

Reviewed by: tjr
MFC after: 2 weeks

Prepare to handle trivial state-dependent encodings. Full support for
state-dependent encodings with locking shifts will come later if there
is demand for it.

Pass NULL instead of a pointer to a zeroed mbstate_t object.

Remove an unused variable: mbresult.

Correctly handle the case where __vfwprintf() fails because it runs out
of memory.

Add implementations of the wprintf() family of functions, which perform
formatted wide-character output.