Add warning and reset flow for non-default masterserver address

For the sake of every brave warrior in #tech-support, watching people blindly flail at their keyboards and fail to port forward.

This adds a prompt to reset masterserver whenever a player attempts to host a netgame or load the in-game Server Browser with a non-default masterserver. If the player mashes through it, the default behavior is to reset, because nobody reads anything and I honestly cannot blame them at all.