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/testlib/showerror.c

@@ -40,7 +40,6 @@ const char *testlib_showerror_symbol;
 void testlib_showerror(const char *file, int line, const char *func,
                        const char *method, const char *symbol, const char *code,
                        char *v1, char *v2) {
-  char *p;
   char hostname[128];
   __stpcpy(hostname, "unknown");
   gethostname(hostname, sizeof(hostname));