Fix warnings

This change fixes Cosmopolitan so it has fewer opinions about compiler
warnings. The whole repository had to be cleaned up to be buildable in
-Werror -Wall mode. This lets us benefit from things like strict const
checking. Some actual bugs might have been caught too.

libc/thread/sem_timedwait.c

@@ -53,7 +53,6 @@ static void sem_timedwait_cleanup(void *arg) {
  */
 int sem_timedwait(sem_t *sem, const struct timespec *abstime) {
   int e, i, v, rc;
-  struct timespec ts;
 
   e = errno;
   for (i = 0; i < 7; ++i) {