Fix NPC positioning in mapgen

[Coolthulhu]

Closes #20654
Closes #20684

Also improved the rotation code, should be easier to understand what is happening.
Well, improved readability - to make it more robust, I made it use a significantly slower method of re-placing NPC - instead of simply moving the NPC, it deletes it from overmap and then places it again, which is linear with total number of NPCs in the overmap buffer. This isn't a big deal though, as NPCs dying also take this much time. If it ever becomes a big deal, I'll optimize it.

Split spawn_at into two functions:

• spawn_at_sm does what spawn_at did before
• spawn_at_precise places a NPC at given submap+position, essentially doing spawn_at+setpos

setpos can no longer be used at mapgen time.

[Toothspit]

Refugee center's fixed; I might be doing something wrong, but this places the starting NPC 3 tiles NW of the evac shelter.

Linux gcc tiles 64 your npc-pos-fix branch here's a save.

[Coolthulhu]

Used wrong coords. Should be fixed now - I placed the NPC closer to the player to make sure all locations are "safe" (ie. that some weird lab config will not move the NPC into a locked closet).
Double checked the rest of the coords - the rest use more specific coords and so should be fine.