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/str/wcschr.c

@@ -23,7 +23,7 @@
  */
 wchar_t *wcschr(const wchar_t *s, wchar_t c) {
   for (;; ++s) {
-    if (*s == c) return s;
+    if (*s == c) return (wchar_t *)s;
     if (!*s) return 0;
   }
 }