pkg fails to register a package with NFS root

While building packages on a PandaBoard with an NFS root pkg failed to register the installation with the error:

    Assertion failed: (!STAILQ_EMPTY(&shlibs)), function shlib_list_find_by_name, file elfhints.c, line 125.

On further investigation I found this was because the `readdir` call in `scan_dirs_for_shlibs` was returning a dirent structure with the `d_type` field set to `DT_UNKNOWN` (= 0) and therefore not adding anything to the `shlibs` list.

When I asked about this on IRC Jilles Tjoelker said it is a valid response as this field is not guaranteed:

    21:08  * _andy_t_ wonders why readdir is returning a dirent with d_type == 0
    21:10 <@jilles> _andy_t_, that's a normal thing for many filesystems such as
                    NFS without the rdirplus option
    21:11 <@jilles> d_type is not guaranteed; if the filesystem would have to
                    perform work equivalent to a full lstat(), it just returns
                    DT_UNKNOWN

To fix this I've changed the check after the `readline` call in `scan_dirs_for_shlibs` to allow for `DT_UNKNOWN`. With this packages are correctly registered.