Normalize calling tcp.shutdown multiple times across OSes

[seanmonstar]

Calling tcp.shutdown(how) multiple times will return different values, depending on the OS. On Linux, it returns Ok(()), on OS X, it returns ErrorKind::NotConnected.

Perhaps this behavior could be made to be the same across platforms. If not, at the least, the docs should call it out.

Noticed from hyperium/hyper#508

/cc @aturon @alexcrichton

[steveklabnik]

Tagging as libs until the decision is made that this is different across platforms.

[nagisa]

I don’t think we can do anything sensible here.

[steveklabnik]

Triage: re-tagging as docs to make mention of this