This change fixes inconsistent error values on
File{Conn,Listener,PacketConn} and File method of Conn, Listener.
Updates #4856.
Change-Id: I3197b9277bef0e034427e3a44fa77523acaa2520
Reviewed-on: https://go-review.googlesource.com/9101
Reviewed-by: Ian Lance Taylor <iant@golang.org>
