convert man pages to sphinx

[David Bremner]

This is the first version actually integrated into the build system.

If people think the results are OK, I think we should move merge
because it has become more difficult than I thought to update the rst
files from man page changes.

Of course we can/should still fix any glaring problems.

One thing that is not here at all is any kind of automation of the
fallback for people without (recent) sphinx.

I also left the nroff man sources in the tree, but if I merge this series 
(or something similar) I plan to delete them.