GNU tar -Holdgnu w/o fakeroot seems to be break dpkg 1.19.1

[Guillem Jover <guillem@debian.org>]

Hi!

I was checking the dpkg 1.19.1 FTBFS on the Hurd, and noticed few
things:

 * GNU tar with -Holdgnu seems broken (?) when packing a fifo, as it
   stores at least the S_IROOT mode bit, which makes GNU tar encode
   the mode data in base-256 (dpkg suppports that just fine, but it's
   still unexpected, and seems incorrect to me). None of the other tar
   formats show this behavior.
 * None of the other dpkg tests fail except for this one.
 * dpkg is now being built w/o fakeroot due to the R³ field set to no.
 * Building the source with fakeroot again makes GNU tar with -Holdgnu
   not store said bit, so it's a possible workaround for a dpkg porter
   build, probably.

So while I could workaround this in the test suite, I think it'd be
better to investigate why GNU tar behaves this way, and possibly fix
it there.

Thanks,
Guillem