remote-viewer: learn '-' as URI for standard input connection file
    
    Some users want the ability to set connection details without writing
    to a file or passing command line arguments.
    
    Learn to read the connection file from standard input for such use
    case.
    
    This allows a parent process to set the connection details without
    intermediary file.
    
    Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Reviewed-by: Victor Toso <victortoso@redhat.com>