debian: Clarify where to find the GPL-2 license

Move text about location of GPL-2 on Debian systems into a Comment
field, and add the stock reference to the gnu.org site in case the
license is not provided with the source code.

debian/changelog

@@ -17,6 +17,7 @@ dpkg (1.18.5) UNRELEASED; urgency=medium
     to https:// if the latter is available (round three). This includes the
     dpkg git repository, copyright format URL and examples in man pages among
     others.
+  * Clarify where to find the GPL-2 license in debian/copyright.
   * Perl modules:
     - Add new CTRL_REPO_RELEASE control block type to Dpkg::Control.
     - Add new CTRL_COPYRIGHT_HEADER, CTRL_COPYRIGHT_FILES and

debian/copyright

@@ -125,6 +125,9 @@ License: GPL-2+
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
  .
+ You should have received a copy of the GNU General Public License
+ along with this program.  If not, see <https://www.gnu.org/licenses/>.
+Comment:
  On Debian systems, the complete text of the GNU General Public License
  can be found in ‘/usr/share/common-licenses/GPL-2’ or in the dpkg source
  as the file ‘COPYING’.