pid1: watch bus name always when we have it

pid1: watch bus name always when we have it

Previously we'd only watch configured service bus names if Type=dbus was
set. Let's also watch it for other types. This is useful to pick up the
main PID of such a service. In fact the code to pick it up was already
in place, alas it didn't do anything given the signal was never received
for it. Fix that.

(It's also useful for debugging)

(cherry picked from commit 1e8b312e5a22538f91defb89cf2997e09e106297)