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.

examples/wall.c

@@ -75,7 +75,6 @@ char *GetHost(void) {
 }
 
 char *GetTime(void) {
-  int64_t t;
   struct tm tm;
   struct timespec ts;
   clock_gettime(0, &ts);