Add ability to send HTTP request when checking SSL/TLS certificates

I have a vendor HTTP server implementation here that issues a verbose log error whenever check_http connects to do a TLS certificate check; the log entry seems to be generated because no HTTP request is issued; check_http disconnects after receiving the certificate and before sending the HTTP request.

This patch introduces a new flag -g, --certget which will prevent this early exit, and instead wait until after the response has been received before exiting (the response it not parsed, except for CRITICAL receive errors).