luci-base: add `responseProgress` callback and `stderr` option

luci-base: add `responseProgress` callback and `stderr` option
for LuCI.request and `fs.exec_direct()`.

This commit adds XHR response progress event option in luci.js and fs.js.
And diagnostics.js will use this new event to update command output.

This commit also adds a `stderr` option in `fs.exec_direct()`, and
requires a patch for `cgi-io`.

Currently `cgi-io` redirects stderr to /dev/null, which makes user
unable to see error messages. The patch adds a `stderr` option,
and if it's set to `1`, redirects stderr to stdout.

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