Bug 9802 - add test case to ensure man pages have correct XML

Bug 9802 - add test case to ensure man pages have correct XML

To test:
* run xt/verify-debian-docbook.t and make sure it's all OK
* modify an xml file in debian/docs, try again, make sure it fails

Signed-off-by: Magnus Enger <magnus@enger.priv.no>
- Running "prove -v xt/verify-debian-docbook.t" loops through all the
  XML files in debian/docs and reports them as being ok.
- Removing a "<" from one of the files makes the test report it as
  an error
- I did not verify that the test actually trips up a package build
  if there is invalid XML.
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>