MFC 311568,311584,312387:
Set MORETOCOME for AIO write requests on a socket.

311568:
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.

311584:
Unbreak lib/libsysdecode after r311568 by decoding MSG_MORETOCOME flag
in msgflags

(Actually, this change excludes MSG_MORETOCOME from being decoded)

312387:
Fix regression from r311568: collision of MSG_NOSIGNAL with MSG_MORETOCOME
lead to delayed send of data sent with sendto(MSG_NOSIGNAL).

Sponsored by: Chelsio Communications