Update debian/copyright for the move of files.

* Move README.shlibdeps and libc6-migration.txt to a new historical
  subdirectory and add a README explaining that the files in this
  directory are of historical interest only and are not included in the
  debian-policy binary package.

* Move the autopkgtest, copyright-format, and debconf_specification
  source files to the top-level directory and the debconf_spec/include
  files to a debconf directory.  Simplify the build system to eliminate
  recursive make, reuse the same XML version file for all files, and use
  the same method of embedding a version in Markdown output for all
  files.
* Fix dependencies so that version.xml is generated before XML
  validation is done on XML source files.

* Remove mention of the libc6 migration paper from the package long
  description.  This is no longer shipped in the debian-policy package.

[dgit --overwrite]

I didn't re-alphabetise the lists for a smaller git diff.

* Add a full copy of the BSD license without advertising clause to the
  <legalinfo> section of the debconf specification, instead of just a
  reference to it.

Noticed by Guillem Jover.

* Update the GPL license statements to the current recommended form,
  which doesn't include a street address for the FSF.  Use all-caps for
  the warranty disclaimer just in case it matters.  Link to the general
  FSF license page to make it easier to find the GPLv2, which is
  technically the license under which these documents are distributed,
  even if the GPLv3 may be used if desired.

* 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.

* Restructure the command list in the debconf specification document to
  pass xmllint, which didn't like including the body of an itemizedlist
  as an XML entity.

Also fix a schema error in the author section introduced by a previous
commit (this document uses a different top-level type than the other
policy documents).

- Add the notice from the main document that the copyright notices are
  incomplete to the other documents with copyright notices.
- Add the Debian Policy Mailing List as an author of the debconf
  specification.
- Replace (incorrect) Software in the Public Interest copyrights with
  copyright notices for the original authors.
- Standardize capitalization of Debian Policy Mailing List in notices.

* Clarify the conflict policy between /path and /usr/path by adding the
  leading slash in front of the first path.  Thanks, Ferenc Wágner.

* Fix tools/policy-bug-report to not fail when bug lists are empty and
  to ignore closed bugs.

* Replace broken CPAN URL in the Perl policy with the current URL, and
  turn this into an in-line link rather than a footnote with the full
  URL.  This hides the URL in the text output, but it's not horribly
  important for the text version and easy to find with a search.

Update Standards-Version of the debian-policy package itself

* Fix formatting of the list of shlibs and symbol file locations.  These
  were mistakenly converted to itemized lists instead of variable lists
  during the DocBook conversion, which produced inferior output.

Change http URLs to https for every resource available over https.

* Use UTF-8 instead of ISO-8859-1 as the character set for the text
  versions of policy documents.

Done by passing the -codepage argument to links.

Add a missing "must."  Add a slash in front of the bare path variable
to make it read better in pure text.

Preserve the historic section anchors in older versions of the
upgrading-checklist by including all four version numbers, although
stop this practice with 4.0.0.

Fix a typo in one of the month names noticed by Guillem Jover.

* Clean up the upgrading checklist a bit:
  - Rewrite the introduction to read a bit more smoothly and mention
    that the Standards-Version value omits the minor patch number.
  - Remove the minor patch number from all the versions except for the
    anomalies that contain normative changes, and note those
    explicitly.
  - Remove some unhelpful section headings and trailing colons in very
    old upgrading checklist entries.
  - Standardize the release date format.

Also bump the package version to 4.0.0.0.

Declare a dependency on the upgrading checklist from policy.xml since
it includes it (although this doesn't seem to work in the build system
properly yet).  Suppress the table of contents in the text version as
well.  Add the upgrading-checklist conversion to the dependencies for
the stamp-build target.