luci-mod-network: diagnostics: add realtime command output

luci-mod-network: diagnostics: add realtime command output

The diagnostics page is using `fs.exec()` to execute command, which blocks
until command exit. Users have to wait for a while to see the result.

When doing IPv6 traceroute, this may cause XHR timeout, if target host
is unreachable. (See issue #7210)

This commit uses new `responseProgress` callback and `stderr` option
added in luci.js and fs.js to update command output.

Signed-off-by: Richard Yu <[email protected]>
Link: https://github.com/openwrt/luci/pull/7920