scope: make TimeoutStopUSec= settable for transient units
core: split out unit bus path unescaping into unit_name_from_dbus_path()
scope: implement reset-failed command
scope: fix state string table
logind: port over to use scopes+slices for all cgroup stuff

In order to prepare things for the single-writer cgroup scheme, let's
make logind use systemd's own primitives for cgroup management.

Every login user now gets his own private slice unit, in which his sessions
live in a scope unit each. Also, add user@$UID.service to the same
slice, and implicitly start it on first login.