improve logging output of event clients & opnional DNS lookup

  - when compiling -DDEBUG_CARES a verbose trace of nameserver lookup is done
  - the log-line 'C-ARES: %s %d|%d Sock %d port %hu' can be used to identify requests in tcpdump pcap files: tcpdump -r out.pcap -w /tmp/filtered.pcap port <%hu output>
  - we log the session context, so outputs of rwho can be connected to loglines