sync TcpStream, UnixStream and UdpSocket functionality

[FrankReh]

A few features have been added to one or the other. This brings them back to parity with each other.

Socket:

pub(crate) fn from_std<T: IntoRawFd>(socket: T) -> Socket

    Socket.from_std is changed from being UdpSocket specific to
    being generic over the IntoRawFd trait.

TcpStream:

pub from_std(std::net::TcpStream) -> TcpStream

UdpSocket:

pub(crate) fn from_socket(Socket) -> UdpSocket

pub fn shutdown(std::net::Shutdown) -> io::Result<()>

unsafe fn from_raw_fd(RawFd) -> UdpSocket

fn as_raw_fd() -> RawFd

UnixStream:

pub fn from_std(std::os::unix::net::UnixStream) -> UnixStream

pub(crate) from_socket(Socket) -> UnixStream

pub fn write_all<T: IoBuf>(mut buf: T) -> BufResult<(), T>

pub fn shutdown(std::net::Shutdown) -> io::Result<()>

unsafe fn from_raw_fd(RawFd) -> UdpSocket

fn as_raw_fd() -> RawFd