Bug#921491: debirf: Broken on usr merged systems

[Hervé Werner]

dpgk-deb also keeps the permissions of the extracted files, so the fix should 
be :
-debirf_exec dpkg -i /var/cache/apt/archives/"$KPKG"
+debirf_exec apt-get install -y binutils xz-utils
+debirf_exec ar x /var/cache/apt/archives/"$KPKG" data.tar.xz
+debirf_exec tar -xp --keep-directory-symlink -f data.tar.xz
+debirf_exec rm -f data.tar.xz