os: make POSIX StartProcess work with chroot again.

Skip directory check in startProcess in the presence of
SysProcAttr.

Fixes #3649.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6297083

src/pkg/os/exec_posix.go

@@ -19,9 +19,10 @@ var (
 )
 
 func startProcess(name string, argv []string, attr *ProcAttr) (p *Process, err error) {
-	// Double-check existence of the directory we want
+	// If there is no SysProcAttr (ie. no Chroot or changed
+	// UID/GID), double-check existence of the directory we want
 	// to chdir into.  We can make the error clearer this way.
-	if attr != nil && attr.Dir != "" {
+	if attr != nil && attr.Sys == nil && attr.Dir != "" {
 		if _, err := Stat(attr.Dir); err != nil {
 			pe := err.(*PathError)
 			pe.Op = "chdir"