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

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

Convert this piece of code to use C11 atomics.

As mentioned before, we should at least aim to have one piece of code in
both user space and kernel space that uses C11 atomics, to get some
coverage. This piece of code can be migrated trivially, so it's a good
candidate.

Register signal 33 explicitly as reserved by real-time library, and
use it by its new name (SIGLIBRT) rather than internal definition
in librt (SIGSERVICE).

Approved by: davidxu, arch

Fix compiler warnings.

atomic_fetchadd_int works on unsigned quantities - change
sigev_generation to be unsigned

MFC after: 1 month

Account for recent changes in namespace.h. Use _pthread_create
instead of pthread_create.

Block all signals in helper threads except those should not be blocked.

Remove stale comments.

1. Always call user callback function in newly created thread, it seems
POSIX implies that the user callback function must be executed in clean
environment.
2. Use newly introduced pthread stubs in libc.

Use a thread pool to process notification if sigev_notify_attributes
is default and caller does not require dedicated thread. timer needs
a dedicated thread to maintain overrun count correctly in notification
context. mqueue and aio can use thread pool to do notification
concurrently, the thread pool has lifecycle control, some threads will
exit if they have idled for a while.

Fix a mutex lock/unlock mismatch.

Forgot to revert to use weak symbols when I was debugging, fix it!

Bring in my initial version of POSIX realtime extension library.
Current the library implements mqueue, timer and aio with SIGEV_THREAD
notification supported.

Earlier version reviewed by: deischen