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/style.css

@@ -82,7 +82,6 @@ object, img {
 .isNOK{ color: #900;}
 .isOK{ color: #090;}
 .ip{padding:8px 16px;margin-bottom: 1em;margin-left: 1em}
-/* read-only values line up with the text inside the input fields */
 .rotext{display:inline-block;padding:8px 16px;margin-bottom: 1em;margin-left: 1em}
 .row {display: flex;}
 .rcol {flex: 90%;}