Honor XDG Base Directory Specification

[rigtorp]

Unison should honor the XDG Base Directory Specification when running on Linux (and probably any posix platform).

Specifically profiles should be stored in $XDG_CONFIG_DIR/unison/* and archives in $XDG_DATA_DIR/unison/*.

For backwards compatibility unison could search old and new location for files, but create new files in the new location.

[bcpierce00]

This sounds like a good idea. Happy to consider a PR...

[xaverdh]

There is a package which should help. Unfortunately I don't know ocaml (im a haskell person), still maybe (if I find the time) I will file a pr at some point in the future.