[bash] Define XDG base directories.

[bash] Define XDG base directories.

XDG base directory standard [1] defines environment variables with
important paths in user home directory. For example, XDG_CONFIG_HOME
points to a directory with configuration files.

Many programs respect the XDG base directory specification. For
example, git looks for it dotfile as either $HOME/.gitconfig or
$XDG_CONFIG_HOME/git/config. Therefore, XDG environment variables
should be defined early during startup.

This commit adds 10-xdg.sh which defines XDG base directory
variables. Prefix '10-' ensures that the files will be sourced by
~/.profile before other files in ~/.config/profile.d.

[1] https://specifications.freedesktop.org/basedir-spec