Some more overmap fixes

[DeltaFire15]

About The Pull Request

#2693 adjusts singularities to use the system contents to track affected, but fails to account for the fact not all contents of system_contents are overmap objects (as opposed to GLOB.overmap_objects).
This PR does similar but safer, while also reinforcing some sanity a bit (current_system was not 100% reliable at being set for anomalies and spawn_ship flooded system_contents with typepaths)

Also makes overmaps firing projectiles a bit less likely to runtime if they are near mapedge or were firing while being moved to nulspace.

This fixes #2701 (Well technically it is a testmerge breaking it but this does that part except without breaking).

Why It's Good For The Game

Fix man good.

Testing Photographs and Procedure

This is tested on local.

Changelog

🆑
code: Singularities now use system contents in a safe way.
fix: Spawn_ship no longer floods system_contents with typepaths.
fix: Overmap firing should be a bit more resilient to turf-related oddities.
/:cl:

[Bokkiewokkie]

At some point we probably should just make an add_ship() that actually handles different types of objects. But this works for now