syscall: add Cloneflags to Linux SysProcAttr.
syscall: add Cloneflags to Linux SysProcAttr.

Also use clone(2) syscall instead of fork().

Fixes issue 6214.

R=golang-dev, bradfitz, dave
CC=golang-dev
https://codereview.appspot.com/13159044

wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old