change exit method in rncp

[kujeger]

resolves issue #959, and aligns the exit method used with that in rnx.

I noticed that rnx uses exit(x) in all instances, whilst rncp always uses RNS.exit(x), but I've opted to only alter the instance after printing, as that was the only change needed to resolve this specific issue.

[markqvist]

That's probably a better way to do it. There's a whole lot of exit signalling circus going on for RNS to exit and clean up clearly on all platforms, and I'm hesitant to touch any of it Cause It Works Now™, but this is just in rncp and not the core RNS exit handler itself, so should be pretty easy to verify everything is OK. Just out of curiosity, how many platforms did you test this on?