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.

html/index.html

@@ -360,6 +360,9 @@ button.danger{background:var(--bad);color:#fff;border-color:transparent}
           <label class="row"><span class="lb" data-i18n="i_ip">IP address</span><input class="in" id="sy-ip" size="15"></label>
           <label class="row"><span class="lb" data-i18n="i_mask">Netmask</span><input class="in" id="sy-mask" size="15"></label>
           <label class="row"><span class="lb" data-i18n="i_gw">Gateway</span><input class="in" id="sy-gw" size="15"></label>
+          <label class="row"><span class="lb" data-i18n="sy_dns1">DNS server</span><input class="in" id="sy-dns1" size="15"></label>
+          <label class="row"><span class="lb" data-i18n="sy_dns2">Second DNS</span><input class="in" id="sy-dns2" size="15"></label>
+          <p class="small mut" id="sy-dnsdhcp"></p>
           <div style="display:flex;gap:10px;margin-top:12px">
             <button class="ctl pri" id="sy-apply" data-i18n="c_apply">Apply</button>
             <button class="ctl" id="sy-dhcp" data-i18n="sy_dhcp" data-i18n-t="sy_dhcp_t">Use DHCP</button>