Add infrastructure to build debian packages.

Add infrastructure to build debian packages.

This commit adds the usual files below debian/ so that two debian
binary packages, liblopsub1 and liblopsub1-dev, can be made with a
command like

	dpkg-buildpackage

The bulk of the recipes in debian/rules employ debhelper(7).

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

v4 -> v5
~~~~~~~~
Avoid the term "all-permissive license" to address copyright concerns
pointed out by Thorsten Alteholz.

v3 -> v4
~~~~~~~~
Next round of feedback from Adam Borowski:
 * soname-less source package
 * Full description for liblopsub-dev
 * debian/changelog: UNRELEASED -> unstable

v2 -> v3
~~~~~~~~
Incorporate more feedback from Adam Borowski:
 * version-gen.sh now works also without a git repository
 * The soname has been dropped from the name of the development package
 * library files are installed in the multiarch dir /usr/lib/$triplet

v1 -> v2
~~~~~~~~
Incorporate review comments from Adam Borowski:
 * Rename package name from lopsub to liblopsub1 to make lintian happy.
 * Append "-1" to the debian version to make it a non-native package.
 * Switch source format from "3.0 (native)" to "3.0 (quilt)".
 * Mention what parts are GPLv3-ed and which are LGPLv3-ed.
 * Adjustments for building a shared library.
 * Squash remaining lintian warnings.

rewrite de/copyright