[BZ 21340] add support for POSIX_SPAWN_SETSID

[BZ 21340] add support for POSIX_SPAWN_SETSID

This patch adds support for the POSIX_SPAWN_SETSID flag.

It was recently accepted by the Austin Group:
http://austingroupbugs.net/view.php?id=1044

Checked on x86_64

	Daurnimator  <quae@daurnimator.com>
	Adhemerval Zanella  <adhemerval.zanella@linaro.org>

	[BZ #21340]
	* posix/Makefile (tests): Add tst-posix_spawn-setsid to list of tests.
	* posix/spawn.h: define POSIX_SPAWN_SETSID flag.
	* posix/spawnattr_setflags.c (ALL_FLAGS): Add POSIX_SPAWN_SETSID to
	valid flags.
	* posix/tst-posix_spawn-setsid.c: Add test for POSIX_SPAWN_SETSID.
	* sysdeps/mach/hurd/spawni.c (__spawni): Implementation of
	POSIX_SPAWN_SETSID.
	* sysdeps/posix/spawni.c (__spawni): Likewise.
	* sysdeps/unix/sysv/linux/spawni.c (__spawni_child): Likewise.
	* NEWS: Add note about POSIX_SPAWN_SETSID support.