manager: when reexecuting try to connect to bus only when dbus.service is around (#6773)

Trying to connect otherwise is pointless, because if socket isn't around
we won't connect. However, when dbus.socket is present we attempt to
connect. That attempt can't succeed because we are then supposed
to activate dbus.service as a response to connection from
us. This results in deadlock.

Fixes #6303

(cherry picked from commit 5463fa0a)