Make more improvements to threading support

- fix rare thread exit race condition on openbsd
- pthread_getattr_np() now supplies detached status
- child threads may now pthread_join() the main thread
- introduce sigandset(), sigorset(), and sigisemptyset()
- introduce pthread_cleanup_push() and pthread_cleanup_pop()

libc/runtime/getpagesize.S

@@ -19,6 +19,8 @@
 #include "libc/macros.internal.h"
 
 //	Returns granularity of memory manager.
+//
+//	@see	sysconf(_SC_PAGE_SIZE) which is portable
 getpagesize:
 	.leafprologue
 	.profilable