std: Add close_{read,write}() methods to I/O

Two new methods were added to TcpStream and UnixStream:

    fn close_read(&mut self) -> IoResult<()>;
    fn close_write(&mut self) -> IoResult<()>;

These two methods map to shutdown()'s behavior (the system call on unix),
closing the reading or writing half of a duplex stream. These methods are
primarily added to allow waking up a pending read in another task. By closing
the reading half of a connection, all pending readers will be woken up and will
return with EndOfFile. The close_write() method was added for symmetry with
close_read(), and I imagine that it will be quite useful at some point.

Implementation-wise, librustuv got the short end of the stick this time. The
native versions just delegate to the shutdown() syscall (easy). The uv versions
can leverage uv_shutdown() for tcp/unix streams, but only for closing the
writing half. Closing the reading half is done through some careful dancing to
wake up a pending reader.

cc #11165