This script locates and installs the 'linux-image-amd64' meta-package
that corresponds to the booted kernel.

This should be done if the meta-package was upgraded to a version that
does not boot on the local machine.  (E.g., because the bootloader
configuration cannot be updated.)

In that case, the unused kernel should be removed, and the meta-package
downgraded to match the kernel in use.  But the matching meta-package
has probably been removed from the Debian archives, making this
impossible to do with a mere 'apt' command.

This script will:

1)  download the correct 'linux-image-amd64' deb from
    snapshots.debian.org

2)  install it via dpkg

    and then

3)  mark it 'held' via dpkg

    to prevent accidental upgrades in the future.

We don't check the gpg signature of the deb, but rely on the https
connection to snapshots.debian.org.