system: configurable hostname (device identity)

Add a device hostname settable from the CLI (`hostname <text>`) and the
System Settings page. The value is sanitized on ingest to JSON-safe
printable ASCII (<=23 chars), stored in a shared __xdata buffer, seeded
to "RTLPlayground" at boot, persisted through the startup-config, and
reported in /information.json. It lives in the common header so other
modules can advertise it (LLDP uses it as the System Name TLV).

html/system.html

@@ -25,6 +25,11 @@
 
     <div id="system-tab" class="tab-content active">
       <h1 data-i18n="sys_heading">System Settings</h1>
+      <div class="row">
+        <div class="lcol"> <label for="hostname" data-i18n="sys_hostname">Hostname:</label></div>
+        <div class="rcol"> <input id="hostname" type="text" maxlength="23" size="20"/>
+          <button onclick="hostSub()" data-i18n="sys_apply">Apply</button></div>
+      </div>
       <div class="row">
         <div class="lcol"> <label for="ip" data-i18n="sys_ip">IP address:</label></div>
         <div class="rcol"> <input id="ip" class="ip" type="text" minlength="7" maxlength="15" size="15"/></div>