make sure a given timeslot can only be opened once
As we will very soon have both osmo-bsc and osmo-mgw opening timeslots on the same E1 lines, let's make sure we fail properly if two clients try to concurrently open the same E1 timeslot.
I just checked the DAHDI source: DAHDI fails any second or further open() of the timeslot device with -EBUSY.
The current implementation actually does the exact opposite: close the old socket/client and give the timeslot to the new client.
We should change this behavior to align with how DAHDI behaves. This way we can then rely on this from higher layer code.
There could be something like F_REOPEN or F_FORCE flag, if desired.