http/https web server support

[mgerb]

If the url starts with http/https send a get request to check if the server is up or not, instead of checking if a tcp socket can be opened. If the server responds with http status < 400 then the server is up.

This would be helpful for reverse proxies. For example, I'm using caddy with virtual hosts to proxy multiple web servers on the same port. The proxy will return http status > 400 if a proxied web server is down.