Add: Console command to list search directories for various things

[nielsmh]

Motivation / Problem

It can be difficult to figure out exactly where to put various files for the game to find them, especially if you're running a headless (dedicated) server and need to upload files by SFTP or similar.

Description

This adds a console command that lists all the search directories for all the interesting game data files, as well as the default locations for save, autosave and screenshot files.

Limitations

Potentially leaks some system information when used over remote console, should this have a protection against that?

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, gs_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

[TrueBrain]

Mostly nitpicks :)