Fix: Only possible to build station next to competitors by using CTRL+click

Fix by checking only for stations owned by the current company when
inspecting if there are multiple adjoining stations to the one being built.

When building next to 2 or more owned stations we don't know which
station should be extended. For other companies' stations that's not a
problem since our station won't merge with theirs anyway.

Calling to BuildStationPart should never have another company's station
as a parameter to attach to unless the client is malicious, so just returning
a generic error in that case.