Add support for relations to match multiple tags. Turned out to be a bit more involve...

Add support for relations to match multiple tags. Turned out to be a bit more involved than expected. The upshot of it all is that both route=hiking and route=foot walking routes are perfectly supported. By default, route=hiking are made, but existing tags aren't converted.

The syntax is slightly inconsistent:
    <tag k="route" v="hiking" vmatch="hiking|foot"/>
on the relation, but
      <match k="route" v="hiking|foot"/>
on the path. I don't think it's a major drama, and easily changed. Also, match/@v is not a regexp, it's just pipe-separated values.
Inspired by NopMap <ekkehart@gmx.de> on potlatch-dev, 14 Feb 2011.