[systemd-devel] Some useful patches

[Hristo Venev]

The first patch moves the user generator directories to XDG_RUNTIME_DIR. This
way the path for the generator directories is always under the manager's
runtime prefix. Consistency and code size.

The second patch moves the transient user units to XDG_RUNTIME_DIR. This way
they are deleted on reboot just like the system transient units and do not take
space in XDG_CONFIG_HOME.

The third patch lets transient units set EnvironmentFiles. It can be used in
systemd-run like this: systemd-run -p EnvirontmentFiles="fileA fileB fileC"
Drop-in files for this are not yet implemented.