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.

test/tool/plinko/plinko_test.c

@@ -63,7 +63,6 @@ void SetUpOnce(void) {
 }
 
 TEST(plinko, worksOrPrintsNiceError) {
-  size_t n;
   ssize_t rc, got;
   char buf[1024], drain[64];
   sigset_t chldmask, savemask;