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/mbrtowc.c

@@ -64,7 +64,7 @@ size_t mbrtowc(wchar_t *wc, const char *src, size_t n, mbstate_t *st) {
   if (n) {
     if (OOB(c, *s)) goto ilseq;
   loop:
-    c = c << 6 | *s++ - 0x80;
+    c = c << 6 | (*s++ - 0x80);
     n--;
     if (!(c & (1U << 31))) {
       *(unsigned *)st = 0;