Merge Debian packaging for release “1.5.8-4”.

Merge Debian packaging for release “1.5.8-4”.

* packaging:
  Finalise release “1.5.8-4”.
  Record copyright information for Emacs startup code.
  Declare conformance to “Standards-Version: 3.9.8”.
  Remove deprecated ‘-k’ option to ‘dh_clean’.
  Upgrade Debhelper requirement to “>= 8”.
  Add build targets recommended by Debian Policy §4.9.
  Place PHONY declarations with their targets.
  Install symlinks to ELisp source files, as per Debian Emacs policy.
  Use ELisp ‘add-to-list’ to modify the ‘auto-mode-alist’.
  Use the Debian function for adding the package to the Emacs load-path.
  Perform startup in Emacs only when the package directory exists.
  Use the defined symbol for the Debian Emacs “flavor”.
  Declare packaging VCS repository URLs.
  Declare upstream homepage.
  Declare compatibility with Emacsen provided by Debian.
  Declare versioned dependency on ‘emacsen-common’.
  Support Debhelper adding dependencies with “${misc:Depends}”.
  Correct an obsolete relation type in a build dependency declaration.
  Document handling of Debian source package.
  Configure Git-Buildpackage for this package's source.
  ...