This change fixes inconsistent error values on Read,
ReadFrom{,UDP,IP,Unix} and ReadMsg{UDP,IP,Unix}.

Updates #4856.

Change-Id: I7de5663094e09be2d78cdb18ce6f1e7ec260888d
Reviewed-on: https://go-review.googlesource.com/8992Reviewed-by: Ian Lance Taylor <iant@golang.org>