Apply GatewayUseSameCredentials logic to RDP file parsing as well

Commit removes GatewayUseSameCredentials logic from cmdline.c, and places it after both cmdline and rpd file have been parsed. This provides proper GatewayUseSameCredentials support for the rdp file.

I have a feeling there are going to be more cases where there is logic in cmdline.c that is not yet replicated for the RDP file (this is my 2nd change that addresses this type of scenario).