(Re)spawn players within 'mapgen_limit'

(Re)spawn players within 'mapgen_limit'

Previously, findSpawnPos() did not take the 'mapgen_limit' setting into account,
a small limit often resulted in a spawn out in the void.
Use the recently added 'calcMapgenEdges()' to get max spawn range through a new
mapgenParams function 'getSpawnRangeMax()'.

Previously, when a player respawned into a world, 'objectpos_over_limit()' was
used as a check, which was inaccurate.
Use the recently added 'saoPosOverLimit()' to get exact mapgen edges.

Also fix default value of 'm_sao_limit_min'.