r510: * Thanks to Benjamin Drieu <benj@debian.org>, dh_installdocs -X now works.

r510:    * Thanks to Benjamin Drieu <benj@debian.org>, dh_installdocs -X now works.
     I had to modify his patch to use cp --parents, since -P spews warnings
     now. Also, I made it continue to use cp -a if nothing is excluded,
     which is both faster, and means this patch is less likely to break
     anything if it turns out to be buggy. Also, stylistic changes.
     Closes: #40649
   * Implemented -X for dh_installexamples as well.
   * dh_clean -X substvars will also work now. Closes: #66890