mapgen: Expand adjust_int_map() to take arbitrary target minimum
adjust_int_map() and adjust_int_map_filtered() in mapgen_utils.[ch] produce values going from zero to their given int_map_max. Expand these to also take an int_map_min, for use cases like the FIXME introduced in #44167.
Since there weren't too many places that use it, I decided to change the existing function and all call sites, rather than try and come up with a name for a new function – this way, those calls where the desired minimum is zero explicitly state that.