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;

src/process/posix_spawn.c

@@ -73,6 +73,10 @@ static int child(void *args_vp)
 		__libc_sigaction(i, &sa, 0);
 	}
 
+	if (attr->__flags & POSIX_SPAWN_SETSID)
+		if ((ret=__syscall(SYS_setsid)) < 0)
+			goto fail;
+
 	if (attr->__flags & POSIX_SPAWN_SETPGROUP)
 		if ((ret=__syscall(SYS_setpgid, 0, attr->__pgrp)))
 			goto fail;