[skip ci] Revert signature simplification of sendmmsg and recvmmsg

[Jan561]

Changes the Iterator Item of XS of recvmmsg back to Item = &'a mut AsMut<I> and of sendmmsg back to Item = &'a AsRef<I>.

This partially reverts #2120. Short explanation using recvmmsg:

pub fn recvmmsg<'a, XS, S, I>(
    // elided...
    slices: XS,
) -> crate::Result<MultiResults<'a, S>>
where
    XS: IntoIterator<Item = I>,
    I: AsMut<[IoSliceMut<'a>]>,
{
    for (i, (slice, mmsghdr)) in slices.into_iter().zip(data.items.iter_mut()).enumerate() {
        let p = &mut mmsghdr.msg_hdr;
        p.msg_iov = slice.as_mut().as_mut_ptr().cast();
        p.msg_iovlen = slice.as_mut().len() as _;
        
        // `slice` gets dropped here! If `slice` was not a pointer, `p.msg_iov` could be dangling now!
    }

    // elided ...
}

CI build: https://cirrus-ci.com/build/6131779443097600 (I'll update this if the commit changes)

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API