syscall: implement Pipe2 on Linux and use it in ForkExec

Fixes #2656.

R=golang-dev, bradfitz, iant, minux.ma
CC=golang-dev
https://golang.org/cl/7062057

src/pkg/syscall/exec_bsd.go

@@ -221,3 +221,17 @@ childerror:
 	// and this shuts up the compiler.
 	panic("unreached")
 }
+
+// Try to open a pipe with O_CLOEXEC set on both file descriptors.
+func forkExecPipe(p []int) error {
+	err := Pipe(p)
+	if err != nil {
+		return err
+	}
+	_, err = fcntl(p[0], F_SETFD, FD_CLOEXEC)
+	if err != nil {
+		return err
+	}
+	_, err = fcntl(p[1], F_SETFD, FD_CLOEXEC)
+	return err
+}