Fix crash in depgraph._remove_pkg

Make sure we don't try to remove a packages twice. This happened in the
past when a package was its own digraph-child.

One way to trigger this is a DEPEND on itself as in the test case.

This bug was reported by slyfox on IRC. In his case the cause for the
self-edge is unknown. Unfortunately the system state is lost.
Always warn about unknown mirrors (bug 501352)
Remove bogus countdown during clean list printing (bug 501296)

Before this patch a fake unmerge countdown would be shown
after an @system package was printed. If the users aborts
at this point, it looked as if some packages were missing
from the clean list compared to the --pretend output.
Merge git://git.overlays.gentoo.org/proj/portage into 2013-12-02