Include pthread.h to make this header standalone (needs pthread_t and others)

Will get us rid of pointless patches in the ports tree. ok guenther@

add a pthread_get_name_np to match pthread_set_name_np.
could be useful in ports.
initial diff by David Carlier some time ago.
ok jca

Move the thread-related .h files to /usr/src/include/, since the
implementation is now spread between libc and librthread. No changes
to the content

ok mpi@