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/calls/splice.c

@@ -36,8 +36,8 @@ static struct Splice {
 } g_splice;
 
 static bool HasSplice(void) {
+  int e;
   bool ok;
-  int e, rc;
   e = errno;
   if (IsLinux()) {
     // Our testing indicates splice() doesn't work as documneted on