fix & revisit silent call feature after the split from osmo-nitb
Using osmo-msc's VTY commands 'subscriber [...] silent-call start [...]' I can initiate a paging and activation of an SDCCH, but 'silent-call stop' seems to have no effect. Fix that.
Also look at the channel type selection from the VTY command, which is no longer possible with a separate osmo-bsc; resolve that by limiting the VTY command (or enabling the feature otherwise?).
Furthermore, look at two '#if BEFORE_MSCSPLIT' in silent_call.c, whether these can be re-enabled using the A-interface information, or #2391.
silent call release doesn't work because the ref counting of the conn is not balanced.
See attached pcap of REF logging from the MSC, where the fsm refcount is never removed.
(The silent call is stopped at packet 16)