This change fixes inconsistent error values on
Set{Deadline,ReadDeadline,WriteDeadline,ReadBuffer,WriteBuffer} for
Conn, Listener and PacketConn, and
Set{KeepAlive,KeepAlivePeriod,Linger,NoDelay} for TCPConn.

Updates #4856.

Change-Id: I34ca5e98f6de72863f85b2527478b20d8d5394dd
Reviewed-on: https://go-review.googlesource.com/9109Reviewed-by: Ian Lance Taylor <iant@golang.org>