NEW: games/gnukem

Hi ports --

Attached is a new port, games/gnukem. Dave Gnukem is an open source
retro-style 2D scrolling platform shooter game.

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pkg/DESCR:
Dave Gnukem is an open source retro-style 2D scrolling platform shooter,
inspired by and similar to Duke Nukem 1 (a famous original 1991 game
that launched the Duke Nukem series). It features an integrated level
editor. (Please note this is not a 'clone' or a 're-make'; it is
probably most akin to a parody.)
---

Works well on amd64.

OK?

~Brian