use systemd dbus StartTransientUnit for unpriv cgroup2

use systemd dbus StartTransientUnit for unpriv cgroup2

If, when init'ing cgroups for a container start, we detect that we
are an unprivileged user on a unified-hierarchy-only system, then we
try to request systemd, through dbus api, to create a new scope for
us with delegation.  Call the cgroup it creates for us P1.  We then
create P1/init, move ourselves into there, so we can enable the
controllers for delegation to P1's children through P1/cgroup.subtree_control.

On attach, we try to request systemd attach us to the container's
scope.  We can't do that ourselves in the normal case, as root owns
our login cgroups.

Create a new command api for the lxc monitor to tell lxc-attach the
systemd scope to which to attach.

Changelog:
 * free cgroup_meta.systemd_scope in lxc_conf_free (Thanks Tycho)
 * fix some indent
 * address some (not all) of brauner's feedback

Signed-off-by: Serge Hallyn <serge@hallyn.com>