socket-util: Emulate recvmmsg() and sendmmsg() on Linux only.

socket-util: Emulate recvmmsg() and sendmmsg() on Linux only.

These functions failed to build on OS X because MSG_WAITFORONE is not
defined there.  There are pitfalls for trying to define our own MSG_*
constants, since it's hard to pick a constant that is not used by the
system already.  Because OVS only uses recvmmsg() and sendmmsg() on
Linux, it seems easiest to just emulate them there.

Reported-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>