Merge from head at r201628.

# This hasn't been tested, and there are at least three bad commits
# that need to be backed out before the branch will be stable again.

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 l

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.

show more ...

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 not

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.

show more ...

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