Added support for the XDG specification

Added support for the XDG specification

The XDG configuration location ($XDG_CONFIG_HOME/putty, or
~/.config/putty) is now prefered over the old ~/.putty location, if the
XDG location already exists. If it doesn't exist, we try to use one of
the old locations ($HOME/.putty, [/etc/passwd home]/.putty, /.putty). If
none of the directories exist, we fall back to ~/.config/putty or
~/.putty, if the XDG_DEFAULT macro is defined or not, respectively. The
PUTTYDIR environment variable remains a definitive override of the
configuration location. This all ensures that the old location is still
used, unless the user explicitly requests otherwise.

The configuration directories are created using the make_dir_path()
function, to ensure that saving the configuration doesn't fail e.g.
because of a non-existent ~/.config directory.