NH4: Track knowledge of door locked/trapped status

Doors known to be trapped are colored cyan, locked red and unlocked
green in that priority.  This can be toggled with the existing mapcolors
option.

Note that the system will never remember door locked/trapped status
incorrectly, only whether the player should know it or not.

Compared to NetHack4, mem_door_l and mem_door_t aren't packed into
mem_bg, instead getting their own bits that I'd reserved when I
implemented mapcolors.  The case when door opening resists no longer
exists in UnNetHack, so the lock/trap bits aren't set in that case.  The
NetHack4 commit alters the argument list of dbuf_set() to accomplish all
of this, whereas this code does all the hard work inside dbuf_set()
itself, side-stepping a lot of boilerplate code in the many places that
it's called.  The NetHack4 commit also includes code for highlighting
peaceful monsters, which will be included in this development branch in
the future.

Track player's knowledge of locked/unlocked doors, draw it onscreen
Alex Smith <ais523@bham.ac.uk>
599c3d156cdb89239ec9b317bb2832290c00d34f