- added a 'no MBF21' compatibility flag.

Turns out that there's a few old maps that have the extended line flags set but not the guard bit that forces their clearance.
Astrostein 1's first map is an example for this.

src/maploader/compatibility.cpp

@@ -169,6 +169,7 @@ static FCompatOption Options[] =
 	{ "explode2",				COMPATF2_EXPLODE2, SLOT_COMPAT2 },
 	{ "railing",				COMPATF2_RAILING, SLOT_COMPAT2 },
 	{ "scriptwait",				COMPATF2_SCRIPTWAIT, SLOT_COMPAT2 },
+	{ "nombf21",				COMPATF2_NOMBF21, SLOT_COMPAT2 },
 	{ NULL, 0, 0 }
 };