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.