Add #include <stdlib.h> for exit definition.

Simple TSD implementation, since some thread-safe libraries want
to use it.

Make thr_self() not return NULL.

Add some threadlib stubs that are used by X:
* thr_create() (stub in libc aborts)
* thr_exit() (stub in libc calls exit())
* thr_yield() (stub in libc does nothing)

Also make the libc thr_self() stu

Add some threadlib stubs that are used by X:
* thr_create() (stub in libc aborts)
* thr_exit() (stub in libc calls exit())
* thr_yield() (stub in libc does nothing)

Also make the libc thr_self() stub not abort.

XXX Should we add thrattr_*() stubs for setting the detach-state?

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Add mutexattr_init(), mutexattr_destroy(), and mutexattr_settype() (recursive
mutexes are used by the X libraries).

Make the libc thr_once() stub functional.

Merge the nathanw_sa branch.