Fixes when changing the player's class in single player games

A few fixes related to changing the player's class in single player (offline) games:

1. Fixed players not being assigned the class they chose through the class selection menu or console when they spawned for the first time. They would need to respawn again before their choice takes effect. This also meant that the player could spawn with a class that's forbidden to the team they joined.

2. Fixed the SetPlayerClass ACS function potentially assigning a random class that's forbidden to the player's team.

I also took the time to clean up some of the existing code and reduce deltas to GZDoom code.