Bugfix: Shutdown cleanly when receiving SIGTERM

Previously, when restic received the SIGTERM signal it would terminate
immediately, skipping cleanup and potentially causing issues like stale locks
being left behind. This primarily effected containerized restic invocations
that use SIGTERM, but could also be triggered via a simple `killall restic`.

This has now been fixed, such that restic shuts down cleanly when receiving
the SIGTERM signal.

https://github.com/restic/restic/pull/4703
