lintian: Use XDG_CONFIG_{HOME,DIRS} for the lintianrc file

Accept (and prefer) XDG based paths for the lintianrc file.  At the
same time, stop supporting "LINTIAN_ROOT/lintianrc".  This patch fixes
the "config" part of #701477.

Signed-off-by: Niels Thykier <niels@thykier.net>