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/pthread_ungarbage.c

@@ -21,7 +21,6 @@
 #include "libc/thread/tls.h"
 
 void _pthread_ungarbage(void) {
-  int i;
   struct Garbages *g;
   struct CosmoTib *tib;
   tib = __get_tls();