`MsgHdr[Mut]` should be `repr(transparent)`

[nerodono]

Since documentation explicitly states

This wraps msghdr on Unix and WSAMSG on Windows

MsgHdr[Mut] should be repr(transparent) over this type, or, at least, provide:

• impl From<MsgHdr[Mut]> for libc::msghdr
• MsgHdr[Mut]::as_hdr_ref() -> &libc::msghdr
• MsgHdrMut::as_hdr_mut() -> &mut libc::msghdr

I can start working on this if it's OK

[Thomasdezeeuw]

I think it makes sense to add #[repr(transparent)]. I'm thinking about any downside to stabilising that, but can't really think of any.

[Darksonn]

Seems fine to me.

[Darksonn]

On the other hand, the actual methods you suggested are problematic as we went through a bunch of effort to remove libc types from the public API, so I'd rather not add them back.

[nerodono]

can you release a patch version with these changes on crates.io?

[Thomasdezeeuw]

Send #636.