Extend Interoperability with Chronyd

Allow user-defined scripts to reconfigure and restart a chronyd service as a workaround to the lack of a clock control facility in chronyd. This brings parity in the supportable use cases of ntpd and chronyd. For example:

[crny]
control_script /usr/share/doc/sfptpd/examples/chrony_clockcontrol.py

See the control_script option in Instance-specific Configuration Options, and Chronyd.