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/runtime/fork.c

@@ -37,6 +37,7 @@ int _fork(uint32_t dwCreationFlags) {
   struct CosmoTib *tib;
   struct PosixThread *pt;
   int ax, dx, tid, parent;
+  (void)parent;
   BLOCK_SIGNALS;
   if (__threaded && _weaken(_pthread_onfork_prepare)) {
     _weaken(_pthread_onfork_prepare)();