sig-monitor: Fix exit in signal handler

Calling exit in signal interrupts wasn't correctly handling
the case where the signal interrupts a thread waiting in the
main loop. This can lead to the binder error report:

    CRITICAL: Can't enter dispatch while in dispatch!

This patch defers the call to exit in a job.

Bug-AGL: SPEC-2907

Signed-off-by: José Bollo <jose.bollo@iot.bzh>
Change-Id: I49c7cca1d229ae957d9ea9bfb8838161ce73a53e