1) Don't forget to set __SERR on __slbexpand() error.
2) Check for __fgetwc() errors using errno. Don't check for __SERR
as PR suggested, it user-visible flag which can stick from previous
functions and stdio code don't check it for this purpose.
PR: 212033
MFC after: 3 days
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)
