dyn PDCH: sysmo-bts/oml.c: add ts_connect_as(), absorbing ts_connect() guts
dyn PDCH: sysmo-bts/oml.c: add ts_connect_as(), absorbing ts_connect() guts

For upcoming dyn PDCH switching, I want to be able to set the pchan dynamically
upon ts_connect() and not continue with OPSTART ACK, but with the dyn PDCH.

Thus recoin ts_connect(ts) to ts_connect_as(ts, pchan, cb) and leave
ts_connect() as a thin wrapper to leave init code unchanged.

Change-Id: I09cc794cb424e17411e608c65f2b68e2f2544e07

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts