"dontvisit" setting for G_SetCustomExitVars (Closes #361)

Adds a third parameter for G_SetCustomExitVars to allow for a warp to happen without actually "completing" the level. This also adds said functionality to Linedef 2 (Custom Exit), triggered by the Slope Skew flag.

This will allow map makers to make a warp system that would be intended to not complete levels. It could also make for "fail exits" that one could fall into...

A specific example where this would really help is in SRB2 The Past, where there is a new quality-of-life feature that allows you to warp back to the hub, but this actually "completes" the level, allowing for players to exploit related unlockables.

This is a GIF showing the feature in action. You can type tphub to list all of the hubs, then type something such as tphub gfz to go to the GFZ Hub.

Example of making a function work with it (Go to MAP01, Skip score tally, and dontvisit):

G_SetCustomExitVars(MAP01, 2, 1)

NOTE FOR WHEN THIS GETS MERGED: Wiki pages for G_SetCustomExitVars and Linedef Type 2 will need to be updated to reflect the changes.