Implement cross-platform path support (for .conf, .cue files)

[dreamer]

Windows-only paths used in .conf and .cue files are a major source of headaches for DOSBox users - it makes it impossible for .conf/.cue files written on Windows to be usable on other platforms (and also the other way around - it is possible to write .conf file usable on Linux, but unusable on Windows).

The solution to this is quite simple:

• convert Windows-paths to point to files on Linux filesystem; in general case it can't be done 100% reliably, but for DOS games (where original files are all case-insensitive already), it certainly can. Test implementation was already written for Boxtron in Python and can be found here: winpathlib.py
• for Linux->Windows - display warning/error whenever .conf file refers to path that is impossible on Windows (e.g. the paths including : character).
• use above-mentioned conversion in following features:
  • path to filesystem passed to MOUNT
  • path to file passed to IMGMOUNT
  • path to individual files in .cue file used for imgmounting

There is one prerequisite though: before implementing this, we need to have framework for unit testing, otherwise it will be hard to handle all corner cases without regressions.

[dreamer]

To quote from other issue:

I have proof of concept ready and so far it works on all GOG games I threw at it (Mortal Kombat 3, Tomb Raider, Carmageddon - all games use CUE+something, except .CUE is named .DAT or .INS and .BIN is sometimes named .GOG and paths inside are windows-only and case-insensitive.

Solution translates windows path separators to linux path separators (in one additional place the translation was missing earlier) and converts case-insensitive paths to case-sensitive ones using POSIX glob function (all paths in CUE files and all paths passed to imgmount command).

New functions I implemented will need to be adjusted to work differently on Windows (and perhaps slight modification for macOS will be necessary).

Branch po/fs-utils-2.