Completely rewrite the build system

* Completely rewrite the build system to use debhelper.
  - Remove all hand-rolled package build rules and let debhelper do the
    package construction.
  - Remove gzip compression from the main build and let debhelper handle
    compression of text files.  This allows debhelper to control the
    gzip flags and do whatever is correct for reproducible builds.
  - The top-level Makefile now has conventional all and install targets
    that build all documents and would install them (not that anyone is
    likely to use the install target other than the packaging).
  - Get the document publication dates from debian/changelog instead of
    the current date for more reproducible builds.
  - Do xmllint validation of the copyright-format and debconf
    specification documents as well.
  - Remove a bunch of old leftovers from the clean and distclean targets
    and make distclean identical to clean.  Stop cleaning editor
    autosave files and patch droppings (this can be destructive).
  - Move all doc-base files into the debian packaging directory.