sched/group: implement POSIX saved-set-UID/GID semantics

Adds tg_suid and tg_sgid fields to task_group_s to complete the
POSIX three-field identity model (real, effective, saved-set).

Updates group_inherit_identity() to propagate the new fields from
parent to child task group on task creation.

Fixes setuid(), setgid(), seteuid(), and setegid() to implement
correct POSIX privilege transition logic:
- Root (euid==0): may set any value; all three IDs updated by setuid/setgid
- Non-root: may only set effective ID to real or saved value; else EPERM

Signed-off-by: Abhishek Mishra <mishra.abhishek2808@gmail.com>

include/nuttx/sched.h

@@ -462,6 +462,8 @@ struct task_group_s
   gid_t   tg_gid;                   /* User group identity                      */
   uid_t   tg_euid;                  /* Effective user identity                  */
   gid_t   tg_egid;                  /* Effective user group identity            */
+  uid_t   tg_suid;                  /* Saved set-user identity                  */
+  gid_t   tg_sgid;                  /* Saved set-group identity                 */
 #endif
 
   /* Group membership *******************************************************/