Set MORETOCOME for AIO write requests on a socket.

[jhb]

Summary

Set MORETOCOME for AIO write requests on a socket.

Add a MSG_MOREOTOCOME message flag. When this flag is set, sosend*
set PRUS_MOREOTOCOME when invoking the protocol send method. The aio
worker tasks for sending on a socket set this flag when there are
additional write jobs waiting on the socket buffer.

Test Plan

• tested nginx 1.8 using aio_write for sockets and found that when an HTTP reply was split across multiple AIO requests, the resulting data was not coalesced into MSS packets the same way as if writev() were used to write all the segments together.

Note that this is imperfect as we might set MORETOCOME and then not
send more data if the second AIO write is cancelled before it is
sent. This can happen in regular sockets if a write is interrupted
as well.

[adrian]

i think it's okay. The fact the lower level behaviour isn't completely defined isn't the fault of this patch :)