`recvmsg` should take `&mut [u8]` instead of `&mut Vec<u8>` for cmsg_buffer

[erenon]

Currently, recvmsg takes a mut cmsg_buffer: Option<&'a mut Vec<u8>> argument. This potentially forces the caller to allocate a right-sized Vec (or keep one around), though recvmsg implementation immediately produces a slice of the vector, and never uses it as a vector.

What's the reason for having Vec on the interface? Could we have a &mut [u8] instead? That'd allow the called to put the buffer onto the stack.

[asomers]

If you look at the commit that introduced the &mut Vec argument, 2643afc, you'll see that it did use cmsg_buffer as a Vec, to set its len. It was a later commit, 482a6b1, that removed the set_len operation. So probably the signature can be changed now. Would you care to prepare a PR and test it?

[erenon]

Possible fix prepared: #2524