Support for ancillary data/control messages in `sendmsg`/`recvmsg` as well as the upcoming `sendmmsg`/`recvmmsg`

[poscat0x04]

The idiomatic way of parsing and constructing ancillary data in C is to use the various macros defined in sys/socket.h (CMSG_FIRSTHDR, CMSG_NXTHDR etc.). Those macros act on msghdr directly. Since we no longer return msghdr in the API, we should serialize/deserialize these messages for the user in the API as well.

[poscat0x04]

See also nix's abstraction over recvmmsg: https://docs.rs/nix/latest/nix/sys/socket/fn.recvmmsg.html