More debian packaging cleanup

[Daniel Kahn Gillmor]

This series should apply after "wrap-and-sort -ast" v2 is applied
(id:20191110173748.25792-5-dkg at fifthhorseman.net).

In this series, i clean up a few things that i noticed from applying
dh_missing to the debian packaging.  In particular, we were failing to
ship notmuch(3) (programmer's manual for libnotmuch) and
notmuch-setup(1) (a symlink or copy of notmuch(1) that is referenced
in notmuch-config(1)).

This doesn't get us to the point of enabling dh 12 cleanly yet (more
elpa-notmuch cleanup might be blocked on #946142 and answers to the
questions i raised in id:87a7887akl.fsf at fifthhorseman.net), but it's
a lot of the way there.

Comments and feedback welcome!

         --dkg