monster group coordinate confusion & npc spawn fix

+ overmap::monsters_at bounds checking is 0 - OMAPX*2 as per current
      usage and monster_group data.
    + spawn_mon: Now implicitly uses square range checking for diffuse groups
    + overmap::is_safe multiplies arguments by 2, as per current usage
    + commented functions to document their differing coordinate arguments
npc spawn fix:
    + game::load_npcs was using rl_dist for bounds checking. Now uses square_dist
square_dist:
    + square dist is square. Always.