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/intrin/pthread_mutex_unlock.c

@@ -37,7 +37,7 @@
  * @vforksafe
  */
 int pthread_mutex_unlock(pthread_mutex_t *mutex) {
-  int c, t;
+  int t;
 
   if (__vforked) return 0;