Bug fix in Dpkg::Path::get_pkg_root_dir() and related changes in dpkg-shlibdeps

Bug fix in Dpkg::Path::get_pkg_root_dir() and related changes in dpkg-shlibdeps

* scripts/Dpkg/Path.pm: Fix behaviour of get_pkg_root_dir() when there's
no DEBIAN subdirectory in none of the parent directories.

* scripts/dpkg-shlibdeps.pl: Handle undef values returned by
get_pkg_root_dir() and complain when it's likely to create a problem (when
a RPATH contains $ORIGIN and when the value of this variable can't be
determined because we don't know what is the root directory of the
temporary tree).