dns lookup partial rewrite

Allow to lookup SRV,NS and MX.
Change the callback args to reflects the changes.

Now all the DNS RRs are passed to the callback (answers, authorities and additional).
With a single request something like that can be done : 

$./sample/dns-example -s _xmpp-client._tcp.jabber.org                                                                                                                                             
EVUTIL_AI_CANONNAME in example = 2
resolving SRV (fwd) _xmpp-client._tcp.jabber.org...
SRV: _XMPp-cliENt._tcP.JAbbEr.OrG -> hermes.jabber.org weight: 30, priority: 30, port: 5222
SRV: _XMPp-cliENt._tcP.JAbbEr.OrG -> hermes6.jabber.org weight: 30, priority: 31, port: 5222
NS: jabber.org -> ns1.jabber.org
NS: jabber.org -> ns1.jeremie.com
NS: jabber.org -> ns2.jabber.org
NS: jabber.org -> ns2.jeremie.com
NS: jabber.org -> ns3.jabber.org
IPv4: hermes.jabber.org -> 208.68.163.221
IPv4: hermes6.jabber.org -> 208.68.163.221