munged option to terminate after daemon is idle for specified duration

[dun]

Add a munged option to terminate after the daemon has been idle for a specified duration of time (maybe --idle-timeout). When combined with systemd socket activation (#83), munged could be started on demand and automatically terminate shortly thereafter, allowing the process to run only when needed.

This option would also be very useful for running munged in the test suite. If a test fails and a munged process running in the background is not terminated, the test suite script will hang at the end. Clearly, this is not desired behavior when the test suite is run in an automated manner.

See #53 for the initial rationale.