This gets rid of the GPDB-specific "QE details" message, that was only sent
once at QE backend startup, to notify the QD about the motion listener port
of the QE backend. Use a standard ParameterStatus message instead, pretending
that there is a GUC called "qe_listener_port". This reduces the difference
between the gp_libpq_fe copy of libpq, and libpq proper. I have a dream that
one day we will start using the standard libpq also for QD-QE communication,
and get rid of the special gp_libpq_fe copy altogether, and this is a small
step in that direction.

In the passing, change the type of Gp_listener_port variable from signed to
unsigned. Gp_listener_port actually holds two values: the TCP and UDP
listener ports, and there is bit-shifting code to store those two 16-bit
port numbers in the single 32-bit integer. But the bit-shifting was a bit
iffy, on a signed integer. Making it unsigned makes it more clear what's
happening.