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/config.js

@@ -25,6 +25,7 @@ const conf_cmds = [
   /^igmp\s+(on|off)$/,
   /^mtu\s+\d{1,2}\s+\d+$/,
   /^bw\s+(in|out)\s+\d{1,2}\s+\S+$/,
+  /^hostname\s+.{1,23}$/,
 ];
 const conf_overwrite = [
   /^ip\b/,
@@ -49,6 +50,7 @@ const conf_overwrite = [
   /^igmp\b/,
   /^mtu\s+\d{1,2}\b/,
   /^bw\s+(in|out)\s+\d{1,2}\b/,
+  /^hostname\b/,
 ];
 
 function parseConf(s){