Ignore T.128 FlowPDU in Share Control header

According to the Microsoft RDP specification, T.128 flow control PDUs should be ignored when reading Share Control headers. (http://msdn.microsoft.com/en-us/library/cc240576.aspx). This patch checks if we got a flow control PDU (length = 0x8000) and advances the stream to ignore the PDU. The flow control PDU specification is available at http://www.itu.int/rec/T-REC-T.128-199802-S/en.
@mdhorda if it's really the case that the padding and the type are received in different order we should add a note in the code that one is aware of this fact.