#455 underworld needs implementation

we have a field in the zones table which isn't even set to the right values.. we can use the map when it loads to define the lowest Y value and from there establish how low players should be able to go.

I don't want to have to re-generate all the maps to add a miny/maxy so we will just have the server while loading the map determine the miny/maxy. This will allow us to set the underworld by maps alone.

this brings R_Zone, UseMapUnderworldCoords into play which by default is 1. This will use the maps interpretation of the underworld.

Alternatively if you want to use the zones table 'underworld' field, which is not set yet, those would be enforced if you set R_Zone, UseMapUnderworldCoords to 0.

making a second rule to add some buffer

    RULE_INIT(R_Zone, MapUnderworldCoordOffset, "-50.0"); // adds (or in the case of negative value subtracts) so that the underworld marker is lower when map is using its lowest Y coordinate