core: Add RootEphemeral= setting

core: Add RootEphemeral= setting

This setting allows services to run in an ephemeral copy of the root
directory or root image. To make sure the ephemeral copies are always
cleaned up, we add a tmpfiles snippet to unconditionally clean up
/var/lib/systemd/ephemeral. To prevent in use ephemeral copies from
being cleaned up by tmpfiles, we use the newly added COPY_LOCK_BSD
and BTRFS_SNAPSHOT_LOCK_BSD flags to take a BSD lock on the ephemeral
copies which instruct tmpfiles to not touch those ephemeral copies as
long as the BSD lock is held.