Create a dummy texture in idx zero if none defined

Fixes spurious HOM doors appearing when using PKE-as-IWAD without any
TEXTUREx lumps present.
Bug fix for SDMG_EXITLEVEL

Case in P_DamageMobj for sector->special == 11 should have been changed
to damageflags & SDMG_EXITLEVEL to prevent possible spurious pistol
starts.
FIXMEs and reformatting only

Mainly issues remaining from the new linedef system conversion.
Compat fix for P_SpawnDoorCloseIn30

If a generalized sector's lower 5 bits are set to 10, BOOM spawns a
30-seconds close door but then obliterates the gen props of the sector,
*including* SECRET and other flags.
Raise in 5 minutes door has same glitch

Hardly surprising. Also, this function has been taking an unused
parameter for more than 10 years.
Started work on new sector special system

Similar to previous line special binding system. Very skeletal right
now.
Refactor hticPushType in sector_t
Use LIGHT_MASK instead of hard-coded 31
Finished up new sector special system

TODO: integration with game engine