Add infrastructure to build debian packages.

This commit adds the usual files below debian/ so that debian source
and binary packages can be made with a command like

    dpkg-buildpackage

The debian/rules file relies on dh but overrides the dh_auto_configure
and the dh_autoreconf targets. The default recipe for dh_auto_configure
results in warnings because tfortune's configure does not implement
some of the options which dh appends to the configure command. The
override for autoreconf is needed because the default recipe would
remove our configure wrapper and replace it with the generated script.

This has been tested on debian-9, debian-10 and debian-11. On all
systems, lintian(1) reports no warnings and no errors.

v3 -> v4
~~~~~~~~
* Fix Bug ID in debian/changelog (pointed out by Adam Borowski)

v2 -> v3
~~~~~~~~
* Switch to a simple debian/rules file as suggested by Alexis Murzeau
  and Adam Borowski.

v1 -> v2
~~~~~~~~
Incorporate feedback from Adam Borowski:
 * Use dh_installchangelogs instead of open-coding it
 * Use gzip -n to avoid rendering the build non-reproducible
 * Fix the installation path of debian/copyright by running dh_install,
 * Create md5sums file