[merge] add 'make dist' and related things, and documentation to use them

[Martin Pool]

* 'make dist' builds a tarball by automating some steps previously
done manually.  This is based on the shell script I've used
previously.
* 'version --short' prints just the version, for use by the above, and
probably useful in other places too.
* 'make check-dist-tarball' runs checks on the packaged code -- and
currently fails on Hardy because of
<https://launchpad.net/bugs/212917>

I'd like to merge this to use it for 1.4

-- 
Martin <http://launchpad.net/~mbp/>
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