Location Updating procedures from both CS and PS overwrite the same
last_lu_seen field of a subscriber. For upcoming D-GSM it will be important to
distinguish those, because only CS attaches qualify for MSISDN lookup.
Add column last_lu_seen_ps, and upon PS LU, do not overwrite last_lu_seen, so
that last_lu_seen now only reflects CS LU.
In the VTY, dump both LU dates distinctively.
Change-Id: Id7fc50567211a0870ac0524f6dee94d4513781ba
07e1602d2d