Update .po and strip down untranslated entries.

This work is done by execute these commands manually:

    $ po4a --copyright-holders="The Rust Project Developers" \
        --package-name="Rust" \
        --package-version="0.10-pre" \
        -M UTF-8 -L UTF-8 \
        doc/po4a.conf
    $ for f in doc/po/**/*.po; do
    >   msgattrib --untranslated $f -o $f.strip
    >   if [ -e $f.strip ]; then
    >       mv $f.strip $f
    >   else
    >       rm $f
    >   fi
    > done

It should be managed by the build system automatically to use in our
translation workflow, but I've not yet done that.

At least one mostly-translated (over 80% translation rate) document is needed to test the translation workflow, so I'll working on Japanese translation.