[#1945] fix building docs from tarball

[#1945] fix building docs from tarball

- Move tools/* EXTRA_DISTs inside tools/Makefile.am because that's where
  they belong.
- Add check-messages.py to EXTRA_DIST in tools/Makefile.am to allow
  someone who downloads the tarball to regenerate the entire message
  documentation just like before.
- Add debug-messages.rst and kea-messages.rst to the tarball so that
  they are not forced to use check-messages.py to regenerate message
  documentation.
- Add singlehtml to all rule. It is the only type of docs that are not
  built by default.