cleanup: fix make distclean in doc

RRD*.pod were in the repository, but were always generated and were
removed by "make distclean", resulting in an unexpected response from
"git status".  Resolve this by removing them from the repository.

Also eliminate the wildcards from CLEANFILES, instead explicitly listing
all derived files.

Also add a couple files to EXTRADIST (PDF documentation and the RRD*.pod
files).

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Merge pull request #473 from pabigot/cleanup-distclean

cleanup: fix make distclean in doc