Add a function to destroy any keys that were set on the current thread
using the algorithm from Dr. POSIX's pthread_key_create. Add some
defines to pthread.h for pthread key use, and implement
pthread_key_delete. It has a prototype in pthread.h, but any program
trying to actually use it would be in for a link-time surprise.
Currently, keys are destroyed either via global destructors, with the
s_key_destroyer object, or in exit_thread. exit_thread is invoked by
pthread_exit, and transitively by pthread_create, via the
pthread_create_helper that ensures all threads created with the pthread
API properly clean up for themselves when they exit gracefully.
A future patch might make s_key_destroyer a C++11 thread_local instead,
assuming we get thread_local and thread_local destructors working.
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