simplify pthread_attr_t stack/guard size representation

previously, the pthread_attr_t object was always initialized all-zero,
and stack/guard size were represented as differences versus their
defaults. this required lots of confusing offset arithmetic everywhere
they were used. instead, have pthread_attr_init fill in the default
values, and work with absolute sizes everywhere.

src/thread/pthread_attr_setstack.c

@@ -4,6 +4,6 @@ int pthread_attr_setstack(pthread_attr_t *a, void *addr, size_t size)
 {
 	if (size-PTHREAD_STACK_MIN > SIZE_MAX/4) return EINVAL;
 	a->_a_stackaddr = (size_t)addr + size;
-	a->_a_stacksize = size - DEFAULT_STACK_SIZE;
+	a->_a_stacksize = size;
 	return 0;
 }