When not finding a .nanorc file in the user's home directory, nano will
look for a nanorc file in $XDG_CONFIG_HOME and in the ~/.config/nano/
fallback directory.  And when not finding a .nano/ subdir in the user's
home directory, nano will look for (or create) the history files in
$XDG_DATA_HOME or in the ~/.local/share/nano/ fallback directory.

This is a partial implementation of the XDG Base Directory Specification:
https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

,
for the purpose of reducing the clutter in a user's home directory, and
to make it easier to back up just the configuration files.
Signed-off-by: Simon Ochsenreither <simon@ochsenreither.de>
Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>