Duke3D: Extend newgamechoices with a third layer of entries AND add NGC commands for controlling flags

Two separate commits, the first extends the newgamechoices menu with a third layer and allows the Usermaps menu to be accessed from any layer.

The second adds a CON command that allows getting, and enabling/disabling the menu flags for the newgamechoices entries. This also means you can unhide an entry without also unlocking it.

Furthermore , the second commit also allows altering the flags for the third layer, which isn't possible with the first commit alone. (I'm not introducing a third parameter for struct members just for this)

Make sure to thoroughly review both commits. The CON commands are unusual in that they take a variable-sized number of arguments.