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/x/xhomedir.c

@@ -25,7 +25,6 @@
  * Returns home directory.
  */
 char *xhomedir(void) {
-  int fd;
   const char *a, *b;
   if ((a = getenv("HOME"))) {
     b = "";