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.

rtlplayground.c

@@ -25,6 +25,7 @@
 #include "machine.h"
 #include "phy.h"
 #include "syslog.h"
+#include "dns.h"
 #include "httpd/page_impl.h"
 #include "boot.h"
 #include "sfp.h"
@@ -1756,6 +1757,7 @@ void main(void)
 	check_and_flash_update_image();
 
 	syslog_init();
+	dns_init();
 
 #ifdef DEBUG
 	// This register seems to work on the RTL8373 only if also the SDS