shared/verbs: show list of verbs when missing

shared/verbs: show list of verbs when missing

Replaces #32062

As discussed in #32062, making 'help' the default verb
is not very appealing for two reasons:

1) If the verb is missing, showing a help which is pages long
   isn't really helpful to locate the problem.
   (https://github.com/systemd/systemd/pull/32062#issuecomment-2064997158)

2) We want to reserve the right to set default verbs to be
   more useful ones, instead of help. E.g. 'busctl' lists all
   bus peers by default.

So, when there are more than 2 verbs, let's instead add
the list of available verbs to the "Command verb required"
message, that serves as a hint. That way we try to be friendlier
to users, but still make the problem obvious.