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/i18n.js

@@ -138,6 +138,8 @@ var LANG = {
     sys_tab_console: 'Console',
     sys_heading: 'System Settings',
     sys_ip: 'IP address:',
+    sys_hostname: 'Hostname:',
+    sys_apply: 'Apply',
     sys_netmask: 'Netmask:',
     sys_gateway: 'Gateway:',
     sys_language: 'Language:',
@@ -317,6 +319,8 @@ var LANG = {
     sys_tab_console: 'コンソール',
     sys_heading: 'システム設定',
     sys_ip: 'IP アドレス:',
+    sys_hostname: 'ホスト名:',
+    sys_apply: '適用',
     sys_netmask: 'ネットマスク:',
     sys_gateway: 'ゲートウェイ:',
     sys_language: '言語:',
@@ -496,6 +500,8 @@ var LANG = {
     sys_tab_console: '控制台',
     sys_heading: '系统设置',
     sys_ip: 'IP 地址:',
+    sys_hostname: '主机名:',
+    sys_apply: '应用',
     sys_netmask: '子网掩码:',
     sys_gateway: '网关:',
     sys_language: '语言:',