Move default data directory to XDG_DATA_HOME on Linux

[facespkz]

This takes the current default into account, so it would only be noticeable on a fresh install.

Also a minor refactor for generating the old path.

[fabiangreffrath]

Thank you!

[facespkz]

Heads up, I think I found a race condition. I don't know how or why it happens, but by all accounts it looks like it shouldn't.

[fabiangreffrath]

What exactly do you mean?

[facespkz]

For some reason, maybe 5-10% of the time, the code will return the old path, regardless of if it exists or not. Maybe a bug with the filesystem I'm on, but I just thought it would wait for the stat() to finish.

[fabiangreffrath]

Hm, you don't check stat()'s return value. On success, it should return 0. If it doesn't return 0, this should be treated just the same as if the directory doesn't exist.

if (stat(base, &data_dir) || !S_ISDIR(data_dir.st_mode))

[facespkz]

After running this about 50 times it looks like it works, so I'll follow this up with another PR.