sys/unix: add Mmsghdr for linux

[nickgarlis]

Expose struct mmsghdr as Mmsghdr, along with SizeofMmsghdr, so
callers can build recvmmsg(2) without redefining the types per
architecture.

This is split out from the Recvmmsg wrapper proposed in
golang/go#80434, since that API is still under discussion and this
change itself is non-controversial.

Updates golang/go#80434

[gopherbot]

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