Bitfields for PC/TCP receive options:
Bit(s)	Description	)
 1	do not remove data from queue, just copy it
 2	do not copy data, just remove it from queue
 5	fail if datagram would be truncated
 6	do not block, return error if no data available
Note:	special case for UDP: if both 1 and 2 set, return number of datagrams