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

debian/apt.auto-removal.sh

@@ -41,7 +41,7 @@ version_test_gt ()
 	return "$?"
 }
 
-list="$(${DPKG} -l | awk '/^ii[ ]+(linux|kfreebsd|gnumach)-image-[0-9]/ && $2 !~ /-dbg$/ { print $2 }' | sed -e 's#\(linux\|kfreebsd\|gnumach\)-image-##')"
+list="$(${DPKG} -l | awk '/^ii[ ]+(linux|kfreebsd|gnumach)-image-[0-9]+\./ && $2 !~ /-dbg$/ { print $2 }' | sed -e 's#\(linux\|kfreebsd\|gnumach\)-image-##')"
 
 latest_version=""
 previous_version=""