hexen: Increase maximum level number to MAP60.

Hexen: Deathkings of the Dark Citadel uses levels up to MAP60, not
MAP40 as used in hexen.wad. Bump up the limit for Hexen to MAP60 so
that it is possible to play multiplayer games with hexdd.wad loaded.

Thanks to Rhiyo for the bug report. This fixes #312.