freerdp-shadow-cli: add support to multi monitor

[efferre79]

Is your feature request related to a problem? Please describe.
RDP clients support multimonitor, currently the shadow server on Linux supports only one monitor creating a screen attached to the primary one.

Describe the solution you'd like
To have a /multimon switch like for the client and to be able to select which monitors are shadowed by the server, e.g. /monitors=0,2 in a three monitor setup (where /multimon without /monitors implies all the remote monitors). A single window on the client side is expected

Describe alternatives you've considered
Right now it's only possible to start multiple shadow servers, listening on different ports, pointing to different monitors by using /monitors=X, this doesn't offer the best user experience

Additional context
nothing

[efferre79]

I have found #7514 but I cannot understand how that works because in the code only one monitor is selected by the shadow server. In fact only one monitor is displayed when connecting with mstsc client with "Use all my monitors for the remote session" flag