Bug#979631: APT::Get::AutomaticRemove::Kernels: also allow keeping N newest

[積丹尼 Dan Jacobson <jidanni@jidanni.org>]

I also hope at least one "No available version in archive" version is
left on the disk,

# apt-show-versions -r -p '^linux-image-[0-9]+\.'
linux-image-5.10.0-1-amd64:amd64/unstable 5.10.4-1 uptodate
linux-image-5.9.0-5-amd64:amd64 5.9.15-1 installed: No available version in archive

Else if there is a botched update to all the updatable kernels, no good
one will be left!