tighten filtering of kernel images in apt.auto-removal

tighten filtering of kernel images in apt.auto-removal

The current filtering matches the names of the image metapackages on the
i386 architecture:

$ dpkg-query -l | awk '/^ii[ ]+(linux|kfreebsd|gnumach)-image-[0-9]/ && $2 !~ /-dbg$/ { print $2 }'
linux-image-3.16.0-4-586
linux-image-586

This results in an extra image package being removed from
APT::NeverAutoRemove, losing the intended effect of keeping the {current,
previous, latest} set of images installed.

Requiring a “.” in the package name tightens the matched package names
to those that are installing a specific version of the image, thus
eliding the meta-packages.

Closes: 772732