shell-completion: propose templates for disable/[re]enable/[re]start

shell-completion: propose templates for disable/[re]enable/[re]start

Templates can be [re]enabled, on their own if the have DefaultInstance set,
and with an instance suffix in all cases. Propose just the template name
ending in @, to underline the instance suffix may have to be appended.

Likewise for start/restart.

This means that sometimes superflous units that one will not really
want to operate on will be proposed, but this seems better than
proposing a very incomplete set of names.

https://bugs.freedesktop.org/show_bug.cgi?id=66912