libutil: Limit readLink() error to only overflows.

Let's not just improve the error message itself, but also the behaviour
to actually work around the ntfs-3g symlink bug. If the readlink() call
returns a smaller size than the stat() call, this really isn't a problem
even if the symlink target really has changed between the calls.

So if stat() reports the size for the absolute path, it's most likely
that the relative path is smaller and thus it should also work for file
system bugs as mentioned in 93002d69fc58c2b71e2dfad202139230c630c53a.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Tested-by: John Ericson <Ericson2314@Yahoo.com>