systemctl: share path lookup between 'cat' and 'edit'

systemctl: share path lookup between 'cat' and 'edit'

'systemctl cat' now works for templates too.

'systemctl edit' does not refuse to edit units that have changed on
disk. That restriction didn't seem useful, actually editing units that
have changed on disk before they are started is very reasonable.

'edit' with instances and templates works again:

Now:

$ build/systemctl edit getty@
Failed to copy /etc/systemd/system/getty@.service.d/override.conf to /etc/systemd/system/getty@.service.d/.override.confdff6290408c86369: Permission denied
$ build/systemctl edit getty@tty3
Failed to create directories for /etc/systemd/system/getty@tty3.service.d/override.conf: Permission denied
$ build/systemctl edit --full getty@tty3
Failed to copy /usr/lib/systemd/system/getty@.service to /etc/systemd/system/.getty@tty3.serviced3d175087e7e439b: Permission denied
Failed to create temporary file for /etc/systemd/system/getty@tty3.service: Permission denied
$ build/systemctl edit --full getty@
Failed to copy /usr/lib/systemd/system/getty@.service to /etc/systemd/system/.getty@.servicea3caad491c0f2f3d: Permission denied
Failed to create temporary file for /etc/systemd/system/getty@.service: Permission denied