Don't you love it when typos ruin everything? :P

Untested on my end, but conceptually sound.

# 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.

Fix cv_usejoystick being reset to None when cv_usejoystick was > NumJoys and changing to already-used joystick

Fix menu where cv_usejoystick.value > I_JoyNum and selecting an unused controller and the "used!" prompt pops up erroneously

* Menu improvements to tell the user that they can't set a controller if it's already active

* Don't save cv_usejoystick/2 because hotplugging changes can be made invalid by next run
* Properly set cv_usejoystick/2 for menu toggling
* Force-disable a player's joystick if setting another player's joystick to the same device
* Properly set cv_usejoystick/2 value of BOTH target player AND other players when hotplugging and unplugging

Update code versions to 2.1.21

See merge request STJr/SRB2!338

# Conflicts:
#	src/d_clisrv.c
#	src/d_netcmd.c
#	src/d_netcmd.h
#	src/dehacked.c
#	src/doomdef.h
#	src/g_game.c
#	src/hu_stuff.c
#	src/lua_baselib.c
#	src/m_menu.c
#	src/sdl/i_system.c
#	src/sdl/sdlmain.h

Block previous versions' config defaults with EXECVERSION consvar

See merge request STJr/SRB2!371