spin: New module. * lib/glthread/spin.h: New file, based on lib/glthread/lock.h and lib/asyncsafe-spin.h. * lib/glthread/spin.c: New file, based on lib/asyncsafe-spin.c. * modules/spin: New file. * doc/multithread.texi (Choosing a multithreading API): Spin locks no longer require POSIX threads. (Gnulib multithreading): Mention the 'spin' module.