Using own captive portal (generate_204) instead of Google settings

[rfc2822]

To set your own captive portal URL instead of the Google defaults:

settings put global captive_portal_https_url 'https://MYSERVER/generate_204'

The /generate_204 URL just needs to return 204:

# nginx
location /generate_204 { return 204; }

The setting won’t survive a reboot, so put it to a boot script.