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/stdio/vcscanf.c

@@ -326,7 +326,6 @@ int __vcscanf(int callback(void *),    //
               } else if (tpdecodecb((wint_t *)&c, c, (void *)callback, arg) !=
                          -1) {
                 if (charbytes == sizeof(char16_t)) {
-                  size_t k = 0;
                   unsigned w = EncodeUtf16(c);
                   do {
                     if ((j + 1) * 2 < bufsize) {