add support for multiple urls for route headers. (#276)

add support for multiple urls for route headers. (#276)

* add support for multiple urls for route headers.

add support in gpx for gpx/rte/link, gpx/rte/url, gpx/rte/urlname.
add support in gpx for gpx/trk/link, gpx/trk/url, gpx/trk/urlname.

Note that the gpx writer can violate the schema when fprint_xml_chain
is used to echo unrecognized elements from input to output.
This is because the gpx 1.0/1.1 schema requires an xsd:sequence of
elements, i.e. the order of the elements is specified, and any
elements output by fprint_xml_chain won't necessarily be in
the correct order with other elements that are specifically
handled in the gpx writer.

By processing additional gpx elements as listed above on read we
prevent them from causing schema violations on write.  However,
other unrecognized elements can still cause schema violations.

* make common link code common in gpx reader.