3428: custom sp mapents, crash fixes, bot support etc

[General]

• add custom mapEnts for each SP map to spmod (3 dm spawns, global intermission :: dynamically loaded)
• fix damage and respawn on SP maps
• scr_testclients now works on SP maps
• intercept and skip a entity visibility check on SP maps (caused a crash when spawning bots)
• move MP-SP logic to module "_Map"
• slightly modify the map-exporter to export original mapEnts and not the custom ones