bus: add the ability for backends to queue to input messages at the same time

We need this so that one incoming kernel message can result in two
high-level bus messages, for the case where we synthesize NameAcquired
and NameOwnerChanged in the same instance.
bus: fix callback index when dispatching kernel messages
bus: synthesize NameLost/NameAcquired from kernel messages on top if NameOwnerChange
rename stdio-bridge to bus-proxyd
systemd-bus proxy service and socket files
systemd-bus-driver

driver

Conflicts:
	Makefile.am