Expose nextgametype, skipstats and nextmapoverdrive as Lua globals and add keepcutscene

Game:

Adds boolean keepcutscene

Forces the game play post-level cutscene despite skipstats.

Alongside it, this merge also adds an additional flag to Custom Exit linedef. (TMEF_KEEPCUTSCENE, 4)

Lua API:

global	accessibility	type
nextmapoverdrive	READ ONLY	int
nextgametype	READ ONLY	int
skipstats	READ ONLY	int
keepcutscene	READ ONLY	bool

Function Update:

G_SetCustomExitVars([int nextmap, [int skipstats], [int nextgametype], [boolean keepcutscene])

Note: int nextgametype parameter is already part of the function. Just wiki is outdated in this case.

Test script:

getCustomExitVars.lua