rdp: Allow specifying a listener fd on the command line

We already have a way for a single RDP client connection to be
passed from a parent process to a child using a combination
of environment variable (RDP_FD) and env var (--env-socket)

This patch allows a bound socket fd (as opposed to a client
connection) to be established in a parent process and provided
to the rdp backend. WSLg uses this to set up an AF_VSOCK
socket for communication between a Windows RDP client and a
weston compositor running under a hypervisor.

Co-authored-by: Hideyuki Nagase <hideyukn@microsoft.com>
Co-authored-by: Steve Pronovost <spronovo@microsoft.com>
Signed-off-by: Hideyuki Nagase <hideyukn@microsoft.com>
Signed-off-by: Steve Pronovost <spronovo@microsoft.com>
Signed-off-by: Brenton DeGeer <brdegeer@microsoft.com>