use apr_parse_addr_port() in ap_set_listener(); this allows IPv6

numeric address strings to be specified (though we wouldn't get
an IPv6 socket for it at the moment)

consolidate the use of APR_ANYADDR and make a note about its
protocol dependence


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87369 13f79535-47bb-0310-9956-ffa450edef68