analyze: port verb dispatching to verbs.[ch] API

Let's unify the code for parsing command line verbs, and reuse the
common verbs.[ch] API in systemd-analyze too.

This adds a couple of error messages when people pass too many
arguments. Moreover thus pushes bus allocation into the verb functions,
which corrects a couple of cases where we previously allocated a bus but
really didn't need to.

Other than that behaviour shouldn't really change.