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

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

Implement SIGEV_THREAD notification for aio_fsync.

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.

1. Fix a race in aio_return.
2. Save and restore syscall errno correctly.
3. Style fix.

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