syscall: add linux specific Getsockopt for Ucred struct
syscall: add linux specific Getsockopt for Ucred struct

SO_PEERCRED on unix domain socket will fill a Ucred struct,
thus linux needs a custom Getsockopt variant.
Fixes issue 3836.

R=golang-dev, rsc, iant
CC=golang-dev
http://codereview.appspot.com/6445104

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go