Add support for specifying the RDP client name

RDP allows clients to specify their name or hostname when connecting. In the case of Windows RDP servers, this name is then exposed via the %clientname% environment variable.

Terminal servers may reject incoming connections when those connections each specify the same client name (and thus appear to come from the same machine), and there are likely other reasons such an option exists. Without such an option, the client name claimed by Guacamole will be the same for all connecting clients, and the %clientname% variable becomes almost useless.