Prevent early interrupt from displaying stack trace

If a user pressed Ctrl-C _really_ early in a hook run, they would get an
ugly stack trace since we don't set up a signal handler until we load
the Overcommit code.

Add a signal handler so that these early interrupts can be gracefully
caught.

Change-Id: I184cdcb3a76802095508d33af01b63f694cfbb83
Reviewed-on: http://gerrit.causes.com/41195
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>