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.
Fixed incorrect values for PDUTypeFlow.

Swapped values for PDU_TYPE_FLOW_RESPONSE and PDU_TYPE_FLOW_STOP to
match the T.128 spec.
Added comment to clarify flow control PDU fields.
Merge pull request #2188 from hopToDev/master

Ignore T.128 FlowPDU in Share Control header