Make improvements

- Clean up sigaction() code
- Add a port scanner example
- Introduce a ParseCidr() API
- Clean up our futex abstraction code
- Fix a harmless integer overflow in ParseIp()
- Use kernel semaphores on NetBSD to make threads much faster

libc/calls/struct/sigaction.h

@@ -20,7 +20,6 @@ struct sigaction { /* cosmo abi */
   uint64_t sa_flags;
   void (*sa_restorer)(void);
   struct sigset sa_mask;
-  int64_t __pad;
 };
 
 sighandler_t signal(int, sighandler_t);