Feature: Engine expiry setting to keep owned engines available for purchase

[2TallTyler]

Motivation / Problem

@andythenorth and others want to roleplay with their favorite trains forever, yet continue the passage of time.

We have "vehicles never expire" for this, but it's too big a hammer with a GRF like Iron Horse.

Description

Extend the vehicle expiry setting to add a third option: Engines that are owned by any company remain available for purchase, while unowned vehicles expire normally.

While I'm renaming things, this setting is about engine expiry, not vehicle expiry. 😃

Limitations

Iterating through all vehicles every time CalcEngineReliability() is called feels expensive. We should probably cache a list of currently-owned engines and update it using a timer, once per economy month (about a minute). I could use some help determining the proper way to do this. 🙂

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[frosch123]

I do not quite understand from the diff, how this works.

What happens when the last vehicle is sold?

• If the engine is past phase 3: does the engine immediately become unavailable?
• If the engine is in phase 3: does the engine immediately drop in reliability?

[2TallTyler]

The behavior of selling the last vehicle is identical to what currently happens if Vehicles never expire is disabled. The specifics of that behavior are a bit beyond my understanding. 🙂