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/fseek_unlocked.c

@@ -38,7 +38,6 @@
  */
 int fseek_unlocked(FILE *f, int64_t offset, int whence) {
   int res;
-  ssize_t rc;
   int64_t pos;
   if (f->fd != -1) {
     if (__fflush_impl(f) == -1) return -1;