Werk #14712: check_http: Consolidate host and proxy settings

The active check check_http, which monitores HTTP services, is configurable via the ruleset Check HTTP service. Before this werk, users could configure three host addresses there: * A hostname or IP address * A virtual host * A proxy server

However, check_http has only two arguments to specify host addresses: -I for the actual address to be contacted and -H for the virtual host. Hence, specifying an address, a virtual host and a proxy server in the ruleset did not make sense. In fact, before this werk, if all three of them were specified, the virtual host was simply ignored.

As of this werk, users can configure two options instead of three: the address to contact (either directly or a proxy server) and a virtual host. In case users select to configure a proxy server, they can optionally provide the corresponding port and credentials, as before.

Furthermore, using a proxy most likely requires the HTTP method CONNECT. This method is now used automatically if a proxy is configured, unless a different method is explicitly selected. Previously, this was only the case for the certificate checking mode, but not for the URL checking mode.

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