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>

rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,d0f/go-zh,d0f/go-zh,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,webfd/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,scirelli/scirelli-go,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,webfd/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,rdp/rogerpack2005-golang,bryanxu/go-zh,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,scirelli/scirelli-go,mhennings/marcohennings-go,d0f/go-zh,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,scirelli/scirelli-go,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof