Add TDB reference counting to ipsp_spd_lookup(). If an output

Add TDB reference counting to ipsp_spd_lookup().  If an output
pointer is passed to the function, it will return a refcounted TDB.
The ref happens when ipsp_spd_inp() copies the pointer from
ipo->ipo_tdb.  The caller of ipsp_spd_lookup() has to unref after
using it.
tested by Hrvoje Popovski; OK mvs@ tobhe@