Make pthread_atfork() track the DSO that called it like atexit() does,
unregistering callbacks if the DSO is unloaded. Move the callback
handling from libpthread to libc, though libpthread still overrides the
inner call to handle locking and thread-library reinitialization.
Major version bump for both libc and libpthread.

verification that this fixes various ports ajacoutot@
asm assistance miod@; ok millert@ deraadt@

Add regress for atexit() vs DSO unloading and start on a similar test
for pthread_atfork()