Fix procedural mine mapgen overwriting exit location

[olanti-p]

Summary

SUMMARY: Bugfixes "Fixed procedural mine mapgen overwriting exit location"

Purpose of change

Fix #1643

Describe the solution

Shuffle to nearest empty_rock omt before starting placement.
This means is some rare cases "bottom" procgenned parts can be disconnected from "top" procgenned parts, but that's better than current situation and should affect starting characters only.

Also fixed off-by-one position of mine shaft ladder to manhole exit.

Describe alternatives you've considered

Porting CleverRaven/Cataclysm-DDA#56555 would be ideal, but it uses mutable overmap specials which we don't have (I'd love to see them ported at some point as well though).

Testing

Teleported around the world, so no new mines generated with their -2 offloading room overwritten.