core: only refuse Type=dbus service enqueuing if dbus has stop job

core: only refuse Type=dbus service enqueuing if dbus has stop job

Follow-up for #27579

In #27579 we refused all StartUnit requests for Type=dbus units
if dbus is not running, which means if dbus is manually stopped,
user can't use systemctl to start Type=dbus units again, which
is incorrect.

The only culprit that leads to the cancellation of the whole
transaction mentioned in #26799 is job type conflict on dbus.
So let's relax the restriction and only refuse job enqueuing
if dbus has a stop job.

To summarize, the case we want to avoid is:

1. dbus has a stop job installed
2. StartUnit/ActivationRequest is received
3. Type=dbus service gets started, which has Requires=dbus.socket
4. dbus is pulled in again, resulting in job type conflict

What we can support is:

1. dbus is already stopped
2. StartUnit is received (possibly through systemctl, i.e. on private bus)
3. Type=dbus service gets started, which will wait for dbus to start
4. dbus is started again, thus the job for Type=dbus service

Replaces #27590
Fixes #27588