Improve pathfinder behaviour for finding road depots (fix #7001; see #6410, #6928, #6929)

[J0anJosep]

This PR tries to fix that:

• Road vehicles can reverse at standard road stops.
• Road vehicles can reverse on some occasions, but not necessarily can reverse when they want to find a road depot. Fix that for NPF.
• Use the max penalty when trying to find a depot when appropriate (for automatic servicing; user-requested find-a-depot won't use the max penalty).

Also, some minor spelling mistakes on comments.

Please, note that this PR needs an accurate review.

Fix #7001
See #6410 #6928 #6929

[J0anJosep]

Road vehicles CAN turn around. But NPF didn't reverse the vehicle when the best depot was found on the reversed dir.
I will change the PR and take this into account.

[LordAro]

Probably a good idea to test this on some large savegames to check for no unexpected changes in behaviour. Maybe an OTTDCoop save?

[J0anJosep]

Even as road vehicles can turn around, I wouldn't check two-way for road vehicles when looking for a depot:

• If road vehicles turn around for finding a depot, why they don't try to turn around when finished loading on a go-through road stop?
• Road vehicles cannot turn around on tunnels or bridges, and that would be an issue if a road vehicle on a bridge tried to find a depot.
• When changing the orders of a vehicle (or just skipping and order), the vehicle controller doesn't check for the opposite direction.
• Trams on straight tracks. A tram on a straight track can reverse if the user asks for it (even when there is no rail to reverse), but it looks ugly. Allowing vehicles to reverse when trying to find a depot would show this ugly detail.

I think that disabling two-way checks (as in the current PR) will simplify the whole issue. But I am not fully convinced of this solution. It would be ok that road vehicles checked the opposite direction in some cases, but I don't know whether it is worth it.

[LordAro]

The commit messages could possibly be improved ("[NPF] NPF..." seems a bit odd), but the code itself looks fine, to my eyes

[nielsmh]

Please fix the commit messages (rebase -i).

The first commit message talks about "road vehicles" when it should be "road stations". (Road vehicles can never be tiles.)

The last commit should be "Docs" rather than "Fix", suggestion: Docs: Fix spelling in some comments

Also try to find a way to not repeat YAPF and NPF in the other commit messages, it looks very silly with it first in a tag and then first word in the sentence.