Fix UDMF FOF flags

This fixes the bugs that were introduced by rearranging the FOF flags for UDMF. In particular:

• The flags for custom FOFs (linedef type 259) are now set properly.
• I added a backwards compatibility layer to ensure that old Lua scripts still work correctly. This required renaming ffloor_t::flags to ffloor_t::fofflags and all the FF_ constants to FOF_. Which is kind of annoying, but I didn't see any way around it. flags and the old FF_ constants are still exposed to Lua, and if you use them, the game will do all the necessary conversion in the background. New scripts should use fofflags and the FOF_ constants for regular flags, bustflags and the FB_ constants for the bustable FOF flags, and busttype and the BT_ constants for the bustable FOF type.

srb2win-udmf.exe