This removes a call to G_DestroyParty from CL_ClearPlayer.
The problem with calling it from there is that the
consoleplayer of a splitscreen is removed first, the local
party is cleared. Then G_LeaveParty assert fails when the
next splitscreen player is removed because the console's
party was already cleared.
Replaces g_splitscreen.c with g_party.cpp. Simplifies
party management functions.
Moves externs out of already bloated doomstat.h and
g_game.h into g_party.h.
Cuts down on globals spam.
