Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Revert the previous change to pthread_once() stub in libc. It is actually
a feature that libstdc++ depends on to simulate the behavior of libc's
internal '__isthreaded' variable. One benefit of this is that _libc_once()
is now private to _once_stub.c.

Requested by: kan

Add an internal _once() method. This works identical to pthread_once(3)
with the additional property that it is safe for routines in libc to use
in both single-threaded and multi-threaded processes. Multi-threaded
processes use the pthread_once() implementation from the threading library
while single-threaded processes use a simplified "stub" version internal
to libc. The libc stub-version of pthread_once() now also uses the
simplified "stub" version as well instead of being a nop.

Reviewed by: deischen, Matthew Fleming @ Isilon
Suggested by: alc
MFC after: 1 week