Overall cleanup.

[denisfa]

@rkitover This is a little bit of cleanup. On #94 we discussed about managing a single config file for all platforms (vbam.ini), so we needed to also include a migration path for vbam.cfg on MacOS and Windows.

We also had some stuff duplicated the FILE_SEP, like ${XDG_CONFIG_HOME}//visualboyadvance-m from the xdg functions. Although it has no problem, since all platforms understand the duplicate FILE_SEP, it was a quick fix, so why not?

Finally, it is always a good practice to set a pointer to NULL after deallocating it. This was missing from the variable home that was the cause of that segfault command line issue.

[rkitover]

All of this looks fine to me. I'm not sure about a .cfg extension on windows, I don't think it ever used that, but that's ok.

Also this change for home does not fix the exit code, I have not yet figured out where that comes from, you can test it like this:

./visualboyadvance-m --help; echo $?

Also on windows / path separators are fine and understood by the POSIX layer, although using the backslashes is preferred.

Will merge this now.