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/libc/intrin/pthread_atfork_test.c

@@ -44,7 +44,6 @@ void *ForceThreadingMode(void *arg) {
 }
 
 TEST(pthread_atfork, test) {
-  int pid;
   __enable_threads();
   SPAWN(fork);
   ASSERT_EQ(0, pthread_atfork(prepare1, parent1, child1));