httpd: move the management VLAN into /vlanlist

Review feedback on #306. The value used to ride in /information.json and
the picker fetched /vlanlist separately, so the page needed both requests
to mean anything. /vlanlist now answers {"mgmt":N,"vlan":[...]} and the
picker reads both from the one response. Both consumers in vlan.js were
taught the new shape, and /information.json no longer carries mgmt_vlan.

The truncation guard now reserves 141 bytes instead of 139: the closing
grew to two bytes with the wrapping object, and the comma in front of a
non-first entry was never counted, so the worst case could land one byte
past outbuf even before this change. char_to_html() does not check.

The comments added on this branch are gone as well, style.css and
system.js both, since these files are served byte for byte.

page_impl.rel stays at DSEG 5, OSEG 0, BSEG 3 and the image reports the
same 10183 bytes of XDATA before and after.

html/vlan.js

@@ -110,7 +110,7 @@ async function loadVlanTable() {
   var resp;
   try { resp = await fetch('/vlanlist'); } catch(e) { return; }
   if (!resp.ok) return;
-  var vlans = await resp.json();
+  var vlans = (await resp.json()).vlan || [];
   for (var i = 0; i < vlans.length; i++) {
     var v = vlans[i];
     var vresp;
@@ -181,7 +181,7 @@ function loadVlanList() {
       sel.style.display = 'none';
       return;
     }
-    var vlans = JSON.parse(this.responseText);
+    var vlans = JSON.parse(this.responseText).vlan || [];
     if (!vlans.length) {
       sel.style.display = 'none';
       return;