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/xbasename.c

@@ -26,8 +26,9 @@
  */
 char *xbasename(const char *path) {
   char *base;
-  path = xstrdup(path);
-  base = xstrdup(basename(path));
-  free(path);
+  char *path2;
+  path2 = xstrdup(path);
+  base = xstrdup(basename(path2));
+  free(path2);
   return base;
 }