reorganize pthread data structures and move the definitions to alltypes.h

this allows sys/types.h to provide the pthread types, as required by
POSIX. this design also facilitates forcing ABI-compatible sizes in
the arch-specific alltypes.h, while eliminating the need for
developers changing the internals of the pthread types to poke around
with arch-specific headers they may not be able to test.

src/thread/pthread_attr_setguardsize.c

@@ -3,6 +3,6 @@
 int pthread_attr_setguardsize(pthread_attr_t *a, size_t size)
 {
 	if (size > SIZE_MAX/8) return EINVAL;
-	a->__guardsize = size - DEFAULT_GUARD_SIZE;
+	a->_a_guardsize = size - DEFAULT_GUARD_SIZE;
 	return 0;
 }