Add randommap command to simplify map rotations.

[emctague]

I suggest adding a randommap command to simplify setting up servers that choose from some variety of maps.

Currently, it's rather annoying to have to edit a bunch of lines to set up a map rotation, especially if you want it to appear random. (I've resorted to writing a python script that scans all pk3s for bsp files, randomizes the list and generates a .cfg file with the randomized rotation.) This command would make it easier to get started hosting a server with a nice variety of maps.

randommap would select a random map out of all maps available to the server (i.e. the same maps one would see when autocompleting the map command) and immediately load said map. To ensure maps are randomized consistently, some vstr trickery could be used by the server operator to set off an endless chain of randomization:

set d1 "randommap; set nextmap vstr d1"
vstr d1

I have implemented this feature on a fork. I would submit a PR, but I figure it's important to create an issue for discussion first.