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.

Makefile

@@ -69,6 +69,8 @@ SRCS = \
 	rtlplayground.c \
 	boot.c \
 	sfp.c \
+	dns.c \
+	dns_cli.c \
 	syslog.c \
 	udp_apps.c