Various updates to the minimal-net platform.

### Better, more seamless support for OS X.

This patch enables automatic route setup and cleanup when starting and stopping the minimal-net target on OS X.

Both IPv4 and IPv6 are supported.

Using the minimal-net target on OS X was absolute hell before I came up with this patch. Now it is painless.

### Implement better idle behavior.

The minimal-net target, as currently written, wakes up the CPU every millisecond to check for packets, and will only react in real-time to input from stdin. If you are running this on a laptop battery, your battery will quickly drain.

This change allows the CPU to idle when there is literally nothing to do while still being responsive to input from `stdin` and/or incoming packets. This fix should signficantly improve performance while significantly improving power usage. Win-win.

Also added `_xassert()` implementation so that the contiki-provided `assert()` macro will work properly when used on this platform.