rdp: split off rdp.h
Refactor some of rdp.c into a header file.
Co-authored-by: Steve Pronovost spronovo@microsoft.com Co-authored-by: Brenton DeGeer brdegeer@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