systemctl: improve readability of start_unit()

systemctl: improve readability of start_unit()

start_unit() is a little tangled.  There's an easy part we can untangle,
then readers can concentrate on the more necessary complexity.

* Derive (method, action, mode) more clearly, as disjoint cases based on
  the command.  Don't rely on action_table[_ACTION_INVALID].target being
  implicitly initialized to NULL.

  verb_to_method() is now only used on one case, but not because I strongly
  object to the implicit "StartUnit" cases.  It's more a syntax problem.
  I think the old code takes me longer to understand, because the call
  comes just above a similar-looking call to verb_to_action(), but the
  results of the two functions are used in different ways.  It also helps
  that the new code ends up having a more regular form, for the 4 different
  cases.

  These changes cost 6 extra lines.

* Add an assertion to confirm that we do not pass mode=NULL.