graphics: Generate the HELP screen programatically.

The original Doom HELP screen showed the default key bindings for the
game's controls. This is not so useful nowadays: at worst it's
outright misleading because the player might not be using the
defaults, but it's also redundant because most modern source ports
allow the key bindings to be changed via the menus, which doubles as a
convenient list of the current key bindings.

Z0k suggested an alternative HELP screen that shows the items and
powerups available in the game. More discussion can be seen in this
thread: http://www.doomworld.com/vb/freedoom/69458-help/