systemctl: allow 'edit' to work on templates again

systemctl: allow 'edit' to work on templates again

This got broken in 9d9dd746d461e4524fb3644b20b8138448d666f2, because a template
is not a valid unit, so the check for being masked failed. Avoid this by
handling templates specially. Fixes #9554.

Also, this improves 'cat' with masked units:

(before) $ systemctl cat foofoofoo@.service
Failed to derive unit name prefix from unit name: Invalid argument
Failed to derive unit name prefix from unit name: Invalid argument
Failed to derive unit name prefix from unit name: Invalid argument
Failed to derive unit name prefix from unit name: Invalid argument
Failed to derive unit name prefix from unit name: Invalid argument
Failed to derive unit name prefix from unit name: Invalid argument
Failed to derive unit name prefix from unit name: Invalid argument
Failed to derive unit name prefix from unit name: Invalid argument
Failed to derive unit name prefix from unit name: Invalid argument
Failed to derive unit name prefix from unit name: Invalid argument

(after) $ build/systemctl cat foofoofoo@.service

In check_triggering_units(), the call to unit_is_masked() is replaced with an
open-coded check. This is a bit unfortunate, but unit_is_masked() now requires
LookupPaths to be initialized, which we don't have or need in this case, so it
seems easiest to just accept this tiny code duplication.