core: add transient units

Transient units can be created via the bus API. They are configured via
the method call parameters rather than on-disk files. They are subject
to normal GC. Transient units currently may only be created for
services (however, we will extend this), and currently only ExecStart=
and the cgroup parameters can be configured (also to be extended).

Transient units require a unique name, that previously had no
configuration file on disk.

A tool systemd-run is added that makes use of this functionality to run
arbitrary command lines as transient services:

$ systemd-run /bin/ping www.heise.de

Will cause systemd to create a new transient service and run ping in it.