Allow manual pages and POD files to be translated.

This uses the optional po4a package to split these files into
PO files for translation, and reassemble afterwards.

Note this creates an extra pot file (po-docs/libguestfs-docs.pot).
We don't (yet) combine this with the main po/libguestfs.pot file.
The 'libguestfs-docs.pot' file included in this commit is not the
real thing, just a short cut down snippet for testing.  The real
thing is created if you update one of the dependent files and
rebuild.

Note also the dummy ja.po, for testing the principles.