install: Add 'fat-install' target

This uses the existing --with-fat-dir option and instead of installing
everything to that directory, creates a normal ikiwiki hierarchy of
files including .mdwn files, whacked release notes html files and all
of the appropriate packages.

One stop shopping for adding the release to the web site.

Attempted to update Releasing file to match.

Signed-off-by: Keith Packard <keithp@keithp.com>