vmspawn: Support RUNTIME_DIRECTORY again

vmspawn: Support RUNTIME_DIRECTORY again

In ccecae0efd ("vmspawn: use machine name in runtime directory path")
support for RUNTIME_DIRECTORY was dropped which makes it difficult to
run systemd-vmspawn in a service unit which doesn't have write access
to the regular /run but should use its own managed RUNTIME_DIRECTORY.
What worked before was --keep-unit --system but we can't use
XDG_RUNTIME_DIR and --user because then --keep-unit breaks which
we need because it can't create a scope as there is no session.
Switch back to runtime_directory which handles RUNTIME_DIRECTORY and
tells us whether we should use it as is without later cleanup or if we
need to use the regular path where we create and delete the directory
ourselves.