Fix bot showing up in NiGHTS stages with a two-pronged approach.

* Sometimes, D_MapChange is passed a mapnum of -1, to complete an existing mapchange. Don't handle botingame checks when that happens.
* Disable charsel on secret level platter when selected map is a NiGHTS stage, since gameplay will be identical across characters (consistent with NiGHTS Attack).

Also:

* Replace IT_DYBIGSPACE hack in M_SetupChoosePlayer, and replace with a more direct workaround system for not being able to select characters before warping.