Create doc directories if they don't exist.

The directories 'doc/wag', 'doc/wpg', and 'doc/wug' are used by the
Makefiles to store the compiled documentation.  However, 'doc/wag' and
'doc/wug' are empty so git ignores them.  Ensure that all three
directories exist by creating them during 'configure'.
Various fixes for 'make tarballs'.

- Exclude .git, .gitignore, etc. from tarballs.

- When checking manifests, strip trailing slashes before sorting, not
  afterwards (otherwise, depending on the locale, the sorted order may
  not be the same.)

- Omit the apparently unused 'checkpkg/mitdb' from the MANIFEST.
Add NEWS for the upcoming release.
Update file 'last revised' dates.