implement new posix_spawn flag POSIX_SPAWN_SETSID

this functionality has been adopted for inclusion in the next issue of
POSIX as the result of Austin Group issue #1044.

based on patch by Daurnimator.

include/spawn.h

@@ -21,6 +21,7 @@ struct sched_param;
 #define POSIX_SPAWN_SETSIGMASK 8
 #define POSIX_SPAWN_SETSCHEDPARAM 16
 #define POSIX_SPAWN_SETSCHEDULER 32
+#define POSIX_SPAWN_SETSID 128
 
 typedef struct {
 	int __flags;