[fix #129] Fix register/de-register bug when process is restarted out-of-band.

Fixes a bug when a process is restarted out-of-band wherby the
original pid was never unwatched and thus would trigger
false-positives by god on noticing process exits.

I am not 100% sure that this doesn't cause problems elsewhere. Not exactly sure how to test and don't know enough about the overall architecture to know what I don't know about how all this works across different process mechanisms.

In any case I'm going to use this on my prod box for a bit and see how it goes.