minor optimization to pthread_spin_trylock

use CAS instead of swap since it's lighter for most archs, and keep
EBUSY in the lock value so that the old value obtained by CAS can be
used directly as the return value for pthread_spin_trylock.

fix more instances of old a_xchg (use new a_swap name)

revert some more spin optimizations that turned out to be pessimizations

optimize contended case for pthread_spin_trylock

initial check-in, version 0.5.0