Add a 'kill process group' feature bound to the 'g' key 

Hello,

The first changeset adds a 'kill process group' feature bound to the 'g' key. It works exactly as the 'kill' feature (F9/k), except that it kills all processes sharing the same PGRP instead of a single process.

The second changeset is purely cosmetic and removes trailing whitespaces (mainly to avoid useless diffs in changesets that actually add some value).

The third changeset fixes a few warnings with GCC 4.8.2 by correctly handling return values (when applicable, meaning that there are still two warnings when handling the return values really makes no sense).

Best regards,