Deprecate G_SetCustomExitVars with G_SetNextMap, Exit map flags, fixes 0 index in G_UnlockCondition

In favor of Exit Level Flags - this merge deprecates G_SetCustomExitVars superseding it with G_SetNextLevel.

	// Supported:
	//	G_SetNextLevel();		[reset to defaults]
	//	G_SetNextLevel(int)		[sets mapexitflags]
	//	G_SetNextLevel(nil, int)	[nextmap override only]
	//	G_SetNextLevel(int, int)	[both of the above]
	//	G_SetNextLevel(int, int, int)	[mapexitflags, nextmapoverride and nextgametype]

Constants:

	// level exit flags
	{"EXITMAP_SKIPSTATS",EXITMAP_SKIPSTATS},
	{"EXITMAP_SKIPCUTSCENE",EXITMAP_SKIPCUTSCENE},
	{"EXITMAP_SKIPSPECIAL",EXITMAP_SKIPSPECIAL},
	{"EXITMAP_SKIPRECORDS",EXITMAP_SKIPRECORDS},
	{"EXITMAP_NOTIMEATTACK",EXITMAP_NOTIMEATTACK},

Custom Exit special in UDMF was also updated to support these new flags. I have a few more ideas for this, but currently I will leave it as it is (beyond any problem that could be found).

These globals from !2632 (merged) (current next):

keepcutscene
skiptally

were removed in favor of introduced flags for Lua:

mapexitflags

Also, this merge fixes index 0 for G_UnlockCondition and missing LUA_HudEnabled for hud_pause in case of out of focus display.

unit_setnextmap.lua