(WSASendMsg): Define.
* fhandler.h (class fhandler_socket): Change definition of recv_internal
and send_internal to take WSAMSG pointer as parameter.
* fhandler_socket.cc (WSAID_WSARECVMSG): Define.
(LPFN_WSARECVMSG): Define.
(WSASendMsg): Declare.
(get_ext_funcptr): New function to fetch address of WSARecvMsg.
(fhandler_socket::recv_internal): Take just a LPWSAMSG parameter.
Change code accordingly. If control information is requested,
fetch address of WSARecvMsg and use that instead of WSARecvFrom.
(fhandler_socket::recvfrom): Change return type to ssize_t as
declared in fhandler.h. Accommodate changes to recv_internal.
(fhandler_socket::recvmsg): Ditto. Make sure that control information
is only requested if system, address family, and socket type support it.
(fhandler_socket::send_internal): Take just a LPWSAMSG parameter
and the flags. Change code accordingly. If control information is
provided, use WSASendMsg instead of WSASendTo.
(fhandler_socket::sendto): Drop useless comment. Accommodate changes
to send_internal.
(fhandler_socket::sendmsg): Ditto. Make sure that control information
is only provided if system, address family, and socket type support it.
* wincap.h (wincaps::has_recvmsg): New element.
(wincaps::has_sendmsg): New element
* wincap.cc: Implement above elements throughout.
* include/cygwin/socket.h (CMSG_ALIGN): Phrase in terms of alignment
of type struct cmsghdr.