runtime: drop CLONE_PARENT

The functionality we want (shared ppid) is implied
by CLONE_THREAD already, and CLONE_PARENT
causes problems if the Go program is pid 1 (init).

See issue 1406 for more details.

Fixes #1406.

R=adg, iant
CC=golang-dev
https://golang.org/cl/3971044

src/pkg/runtime/linux/thread.c

@@ -238,8 +238,7 @@ runtime·newosproc(M *m, G *g, void *stk, void (*fn)(void))
 	/*
 	 * note: strace gets confused if we use CLONE_PTRACE here.
 	 */
-	flags = CLONE_PARENT	/* getppid doesn't change in child */
-		| CLONE_VM	/* share memory */
+	flags = CLONE_VM	/* share memory */
 		| CLONE_FS	/* share cwd, etc */
 		| CLONE_FILES	/* share fd table */
 		| CLONE_SIGHAND	/* share sig handler table */