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.

doc/dns.md

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+# DNS
+
+The switch can resolve host names to IPv4 addresses, for the NTP client and
+for anything else that is given a name instead of an address.
+
+```
+dns server <ip> [<ip>]   # one or two servers, 0.0.0.0 removes them
+dns lookup <name>        # resolve a name and print the address
+dns                      # show the servers
+```
+
+Without a configured server the resolver uses the one handed out by DHCP.
+Only address records are looked up; a query goes to the servers in turn,
+every two seconds, and gives up after six tries. A name that already is a
+dotted address is taken as it is, without a query.
+
+The DNS servers are set in the Network card of the System page as well.
+Settings made there or on the console are kept only once the configuration is
+saved.