ports: show the devices connected behind each port

Adds a Connected devices column to the Port Configuration table, fed by
the shared walkL2() helper. A port with a single MAC behind it shows the
MAC directly; several are shown as a count with the full list in the
cell tooltip, since picking one of many to display would suggest it was
"the" device. The walk restarts 15 seconds after each completion and
starts 3 seconds after page load to stay clear of the initial status and
MTU requests. A walk that ends without ok keeps the previous cells,
since an incomplete table would blank ports that have devices behind
them. Translations for the three languages included.

html/i18n.js

@@ -37,6 +37,8 @@ var LANG = {
     port_col_speed: 'Current Link Speed',
     port_col_set_speed: 'Set Speed',
     port_col_disabled: 'Disabled',
+    port_col_devices: 'Connected devices',
+    port_devices: 'devices',
     port_col_apply: 'Apply',
     port_mtu_heading: 'Configure Maximum Frame Size (MTU) forwarded at Port',
     port_auto: 'Auto',
@@ -224,6 +226,8 @@ var LANG = {
     port_col_speed: '現在のリンク速度',
     port_col_set_speed: '速度設定',
     port_col_disabled: '無効',
+    port_col_devices: '接続デバイス',
+    port_devices: 'デバイス',
     port_col_apply: '適用',
     port_mtu_heading: 'ポートの最大フレームサイズ (MTU) 設定',
     port_auto: '自動',
@@ -411,6 +415,8 @@ var LANG = {
     port_col_speed: '当前链路速率',
     port_col_set_speed: '设置速率',
     port_col_disabled: '禁用',
+    port_col_devices: '已连接设备',
+    port_devices: '台设备',
     port_col_apply: '应用',
     port_mtu_heading: '配置端口转发的最大帧大小 (MTU)',
     port_auto: '自动',