Merge remote-tracking branch 'remotes/SRB2/vanilla-hotplug-stableid' into gamepad-tweaks

# Conflicts:
#	src/command.c
#	src/command.h
#	src/g_game.c
#	src/m_menu.c
#	src/sdl/i_system.c
#	src/sdl/i_video.c
#	src/sdl/sdlmain.h

This is NOT just a normal merge commit. I have adjusted the hotplugging code from stableid to account for the last two players, and also removed new 2.1 additions that were deemed irrelevant to Kart.