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.

cmd_parser.c

@@ -19,6 +19,7 @@
 #include "sfp.h"
 #include "dhcp.h"
 #include "syslog.h"
+#include "dns.h"
 #include "uip/uip.h"
 #include "version.h"
 
@@ -1802,6 +1803,8 @@ void cmd_parser(void) __banked
 			parse_mtu();
 		} else if (cmd_compare(0, "syslog")) {
 			parse_syslog();
+		} else if (cmd_compare(0, "dns")) {
+			dns_parse();
 		} else if (cmd_compare(0, "ip")) {
 			if (cmd_compare(1, "dhcp")) {
 				dhcp_start();