Perform a 'systemctl --system daemon-reload' when needed

If a custom unit file is added to /etc/systemd/system, or the given unit
has been modified since the last time systemctl --system daemon-reload
was run, perform the daemon-reload.
Use helper function to build unit name

_canonical_unit_name() already appends '.service' to a non-canonical
unit name, no need to duplicate efforts here.
Merge pull request #4229 from terminalmage/issue2908

Perform a 'systemctl --system daemon-reload' when needed