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 previous commit: We should only mask the top bit in the STRTOG_NaNBits
case, not the STRTOG_Normal case.

Fix strto{f,d,ld}() so they don't return signaling NaNs. C99
says they are never supposed to, and the fact that they did could
cause apps that run with unmasked FP exceptions to SIGFPE after a
scanf() or strtod(). The vendor stated that he will not be fixing
this, citing portability concerns.

This commit was generated by cvs2svn to compensate for changes in r165743,
which included commits to RCS files with non-trunk default branches.

Import of the latest gdtoa sources, which include fixes for minor
problems relating to NaNs and rounding.

Initial import of David Gay's gdtoa library for conversion between
strings and floating point.