Make overriding special stages optional for custom exits

After nextmapoverride was changed to always override special stages, some users reported some regressions related to custom exits. This branch fixes emerald tokens being eaten, and makes overriding special stages optional:

• G_SetCustomExitVars can take an optional boolean argument to override special stages.
• Linedef type 2 has a flag to do the same.

Fixes #933 (closed) and #985 (closed). Supersedes !1910 (merged).

srb2win_specialoverride.exe - Updated 14/03/2023