libmsc ctrl interface: add CTRL command to query a single subscriber's attached state
the osmo-gsm-tester uses the CTRL 'subscriber-list-active' command to list all active subscribers, to then determine whether a given IMSI is attached. That's suboptimal, rather have a CTRL command that queries a given IMSI and returns the LAC if attached, zero otherwise. Alternatively have a query to return various values on a given IMSI (does it exist yet?) and use this in the osmo-gsm-tester instead.