Simplify signature of ProcessStartupPacket()

Simplify signature of ProcessStartupPacket()

There is now only one caller of ProcessStartupPacket().  Let's simplify
the routine so as the GSS and SSL states are tracked inside it.   If
future callers are added, there is less guessing to do.

Suggested-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/aga7lCWluyc5zLb5@paquier.xyz