don't call pager in non-terminals for changelog

Most pagers are nice and default to running non-interactively if they
aren't connected to a terminal and we relied on that. On ci.debian.net
the configured pager is printing a header out of nowhere though, so if
we are printing to a non-terminal we call "cat" instead.

In the rework we also "remove" the dependency on sensible-utils in sofar
as we call some alternatives if calling the utils fail.

This seems to be the last problem preventing a "PASS" status on
ci.debian.net, so we close the associated bugreport.

Closes: 755040

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