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/timespec_tonanos.c

@@ -22,8 +22,9 @@
 /**
  * Converts timespec to scalar.
  *
- * This function will detect overflow in which case `INT64_MAX` or
- * `INT64_MIN` may be returned. The `errno` variable isn't changed.
+. * This returns the absolute number of nanoseconds in a timespec. If
+ * overflow happens, then `INT64_MAX` or `INT64_MIN` is returned. The
+ * `errno` variable isn't changed.
  *
  * @return 64-bit integer holding nanoseconds since epoch
  */