Parse one --link option to add to <rs:ln> links.

    Input string of the form: rel,href,att1=val1,att2=val2

def parse_link(link_str):
    """Parse one --link option to add to <rs:ln> links.

    Input string of the form: rel,href,att1=val1,att2=val2
    """
    atts = {}
    help_str = "--link option '%s' (format rel,href,att1=val1...)" % (link_str)
    try:
        segs = link_str.split(',')
        # First segments are relation and subject
        atts['rel'] = segs.pop(0)
        atts['href'] = segs.pop(0)
        if (atts['href'] == ''):
            raise ClientFatalError("Missing uri in " + help_str)
        # Remaining segments are attributes
        for term in segs:
            (k, v) = term.split('=')
            if (k == '' or v == ''):
                raise ClientFatalError(
                    "Bad attribute (%s) in " %
                    (term) + help_str)
            atts[k] = v
    except ValueError as e:
        raise ClientFatalError("Bad component of " + help_str)
    except IndexError as e:
        raise ClientFatalError("Incomplete component of " + help_str)

    return(atts)