Improve mapvoting keeptwo

Improve mapvoting keeptwo

Renamed the cvars from *_keeptwotime to *_reduce_time.
Allowed keeping more than just two maps, by setting *_reduce_count (new cvar).
If reduce_count is < 2, it will keep every map that received at least one vote, provided there's at least 2 of them.
Improved cvar descriptions.
Cleaned up associated code somewhat.
Improved associated code documentation.