Use XDG base directories by default on Unix systems

[liff]

See XDG Base Directory Specification .

Chooses userdir as follows:

1. Use ~/.visualvm if it already exists (so as to not break existing usage),
2. use $XDG_DATA_HOME/visualvm if XDG_DATA_HOME environment variable is defined and otherwise
3. use ~/.local/share/visualvm.

Similarly for cachedir:

1. Use ~/.cache/visualvm if it already exists or if XDG_CACHE_HOME environment variable is not defined, but if it is
2. use $XDG_CACHE_HOME/visualvm.