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

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

Don't compare thread pointers again.

PS_DEAD state needs not be checked because _thr_find_thread() has already
checked it.

Convert thread list lock from mutex to rwlock.

Avoid various shadowed variables. libthr is now almost WARNS=4 clean except
for some const dequalifiers that needs more careful investigation.

Ok'ed by: davidxu

Non-portable functions are in pthread_np.h, fix compiling problem.

if passed thread pointer is equal to current thread, pass -1 to kernel
to speed up searching.

Implement functions pthread_getaffinity_np and pthread_setaffinity_np to
get and set thread's cpu affinity mask.