Let OMSPEC_FREQ be a world option setting

Working on getting denser maps and the current 72 limit on overmap specials should be user adjustable. Right now I have to run mapgen in a compiled version with this value set to 10 and copy the save file over. Let players lower it.

From omdata.h

// Overmap specials–these are “special encounters,” dungeons, nests, etc.
// This specifies how often and where they may be placed.

// OMSPEC_FREQ determines the length of the side of the square in which each
// overmap special will be placed. At OMSPEC_FREQ 6, the overmap is divided
// into 900 squares; lots of space for interesting stuff!
#define OMSPEC_FREQ 15

Change it, do some testing, and make a PR. There’s no good reason to make this tunable, it’ll just lead to a ton of bug reports where the cause is unclear.