socket-util: Introduce emulation and wrapper for recvmmsg().

socket-util: Introduce emulation and wrapper for recvmmsg().

Not every system will have recvmmsg(), so introduce compatibility code
that will allow it to be used blindly from the rest of the tree.

This assumes that recvmmsg() and sendmmsg() are either both present or
both absent in system libraries and headers.

CC: Yi Yang <yangyi01@inspur.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>