Revision 252793

MFC r242251, r242311:

 Defer sending an independent window update if a delayed ACK is pending
 saving a packet.  The window update then gets piggy-backed on the next
 already scheduled ACK.

MFC r242252:

 Prevent a flurry of forced window updates when an application is
 doing small reads on a (partially) filled receive socket buffer.

 Normally one would a send a window update every time the available
 space in the socket buffer increases by two times MSS.  This leads
 to a flurry of window updates that do not provide any meaningful
 new information to the sender.  There still is available space in
 the window and the sender can continue sending data.  All window
 updates then get carried by the regular ACKs.  Only when the socket
 buffer was (almost) full and the window closed accordingly a window
 updates delivery new information and allows the sender to start
 sending more data again.

 Send window updates only every two MSS when the socket buffer
 has less than 1/8 space available, or the available space in the
 socket buffer increased by 1/4 its full capacity, or the socket
 buffer is very small.  The next regular data ACK will carry and
 report the exact window size again.

 Reported by:	sbruno
 Tested by:	darrenr
 Tested by:	Darren Baginski
 PR:		kern/116335