r12035+: make stop public on shared halt also changes other players' tiles

[Nazalassa]

Title says all. With the patch, when a player makes a shared halt public, only their tiles turn public. Ownership is updated accordingly.

Note: I was not sure what counts towards a player having ownership in a halt: having buildings/platforms, or just tracks, or wayobjs? The patch assumes only buildings/platforms count. If otherwise, set "tiles_converted" to "true" in haltestelle_t::change_owner() whenever needed.