use restrict everywhere it's required by c99 and/or posix 2008

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

src/thread/sem_timedwait.c

@@ -6,7 +6,7 @@ static void cleanup(void *p)
 	a_dec(p);
 }
 
-int sem_timedwait(sem_t *sem, const struct timespec *at)
+int sem_timedwait(sem_t *restrict sem, const struct timespec *restrict at)
 {
 	while (sem_trywait(sem)) {
 		int r;