select kernels to protect from autoremove based on Debian version

This is basically a rewrite of the script with the general idea of
finding the Debian version of the installed kernels – as multiple
flavours will have the same Debian version – select the two newest of
them and translate them back to versions found in package names.

This way we avoid e.g. kernel and kernel-rt to use up the protected
slots even through they are basically the same kernel (just a different
flavour) so it is likely that if kernel doesn't work for some reason,
kernel-rt will not either.

This also deals with foreign kernel packages, kernels on hold and partly
installed kernels (in case multiple kernels are installed in the same
apt run) in a hopefully sensible way.

Closes: 787827

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