Add support for per-map fog color

[ClobberXD]

This would allow maps with custom skyboxes to set an appropriate fog colour. Currently, this is hard-coded to 0xFFFFFFFF in the set_sky call (in ctf_map_core). This results in the moon map having white fog in spite of the lack of atmosphere, which breaks immersion and impairs visibility in the dark.

I'm thinking of a new fog_color field in map.conf which would be retrieved if skybox exists.

It isn't necessary to code it in for v2.0.0 - this can come later too.