Align the behavior of osmo-e1d with that of DAHDI: If a timeslot is
opened once, it cannot be opened again by anyone until it is closed
by the current owner. This way we'd have the same failure semantics
in DAHDI vs. osmo-e1d, which is very useful in case of misconfiguration
when osmo-bsc + osmo-mgw would "fight" over a timeslot.
Add a osmo_e1dp_client_ts_open_force() function that allows to override
and get back the original behavior.