sd-bus: drop references to legacy /var/run D-Bus socket

sd-bus: drop references to legacy /var/run D-Bus socket

Let's directly reference /run instead, so that we can work without /var
being around, or with /var/run being incorrectly set up.

Note that we keep the old socket path in place when referencing the
system bus of containers, as they might be foreign operating systems,
that still don't have adopted /run, and where it makes sense to use the
standardized name instead. On local systems, we insist on /run being set
up properly however, hence this limitation does not apply.

Also, get rid of the UNIX_SYSTEM_BUS_ADDRESS and
UNIX_USER_BUS_ADDRESS_FMT defines. They had a purpose when we still did
kdbus, as we then had to support two different backends. But since
that's gone, we don't need this indirection anymore, hence settle on a
one define only.