dns: resolve host names

A small resolver for IPv4 address records, so that clients such as NTP can
be given a host name: "dns server <ip> [<ip>]", "dns lookup <name>" and
"dns". Without a configured server it uses the one DHCP handed out. A query
goes to the servers in turn every two seconds and gives up after six tries;
answers with compressed names and CNAME records ahead of the address are
understood. A name that already is a dotted address is taken without a
query.

The resolver sits in BANK3, the command line in BANK2 next to the helpers it
uses. The System page shows and sets the servers.

udp_apps.c

@@ -6,4 +6,5 @@ void udp_callbacks(void)
 {
 	dhcp_callback(uip_udp_conn->lport); 	// let the application decide if this is for it or not
 	syslog_callback(uip_udp_conn->lport);	// let the application decide if this is for it or not
+	dns_callback(uip_udp_conn->lport);
 }