core: remove support for API bus "started outside our own logic"

core: remove support for API bus "started outside our own logic"

Looking at a recent Bad Day, my log contains over 100 lines of

    systemd[23895]: Failed to connect to API bus: Connection refused

It is due to "systemd --user" retrying to connect to an API bus.[*]  I
would prefer to avoid spamming the logs.  I don't think it is good for us
to retry so much like this.

systemd was mislead by something setting DBUS_SESSION_BUS_ADDRESS.  My best
guess is an unfortunate series of events caused gdm to set this.  gdm has
code to start a session dbus if there is not a bus available already (and
in this case it exports the environment variable).  I believe it does not
normally do this when running under systemd, because "systemd --user" and
hence "dbus.service" would already have been started by pam_systemd.

I see two possibilities

1. Rip out the check for DBUS_SESSION_BUS_ADDRESS entirely.
2. Only check for DBUS_SESSION_BUS_ADDRESS on startup.  Not in the
   "recheck" logic.

The justification for 2), is that the recheck is called from unit_notify(),
this is used to check whether the service just started (or stopped) was
"dbus.service".  This reason for rechecking does not apply if we think
the session bus was started outside our logic.

But I think we can justify 1).  dbus-daemon ships a statically-enabled
/usr/lib/systemd/user/dbus.service, which would conflict with an attempt to
use an external dbus.  Also "systemd --user" is started from user@.service;
if you try to start it manually so that it inherits an environment
variable, it will conflict if user@.service was started by pam_systemd
(or loginctl enable-linger).