bug#583: Use XDG basedir spec for configuration files?

[Fernando]

The XDG Base Directory Specification provides some common paths in user's home to store personal application configurations, or, using their own words "defines where these files should be looked for by defining one or more base directories relative to which files should be located".

XDG Base Directory Specification allows efficient backup, in which you can easily choose to backup your data and/or your configuration files for instance. An application could easily propose such backup for the whole system if all applications were matching these specification.

Therefore, although I think that this bug/enhancement is not vital, it would be great for Emacs to be "FreeDesktop XDG Base Directory Specification" compliant. 

Currently Emacs is using "$HOME/.emacs.d/". This is what XDG basedir spec defines:

• $XDG_DATA_HOME (usually $HOME/.local/share/) as "the base directory relative to which user specific data files should be stored"
• $XDG_CONFIG_HOME (usually $HOME/.config/) as "the base directory relative to which user specific configuration files should be stored"
• $XDG_CACHE_HOME (usually $HOME/.cache/) as "the base directory relative to which user specific non-essential data files should be stored"

(http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html)

In order to make Emacs XDG basedir compliant, I think that it would be nice to use:

• $XDG_CONFIG_HOME/emacs  ;;for "init.el" and storing customize configuration

• $XDG_DATA_HOME/emacs  ;;for sessions and backups
  

• $XDG_CACHE_HOME/emacs  ;;for cache files
  

Or maybe some other distribution. What do you think about it?

There's a small C library that may be useful check it out, written for the sole purpose of xdg basedir spec compliance: https://n.ethz.ch/student/nevillm/download/libxdg-basedir/