package/skeleton-init-sysv: Set sticky bit on /dev/shm

/dev/shm is a world-writable directory, like /tmp, and should also
have the sticky bit set.  Without this, any user can delete and
replace another user's files in /dev/shm.

This bug has been present since /dev/shm was added to the skeleton
/etc/fstab, but appears to have been fixed for systems using systemd
by commit 76fc9275f14e "system: separate sysv and systemd parts of the
skeleton" which went into Buildroot 2017.08.

Signed-off-by: Ben Hutchings <ben.hutchings@mind.be>
Fixes: 22fde22e35f98f7830c2f8955465532328348cd1
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 0b2967e15800421efbdfe3a7a6061cf6bd84134d)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/skeleton-init-sysv/skeleton/etc/fstab

@@ -2,7 +2,7 @@
 /dev/root	/		ext2	rw,noauto	0	1
 proc		/proc		proc	defaults	0	0
 devpts		/dev/pts	devpts	defaults,gid=5,mode=620,ptmxmode=0666	0	0
-tmpfs		/dev/shm	tmpfs	mode=0777	0	0
+tmpfs		/dev/shm	tmpfs	mode=1777	0	0
 tmpfs		/tmp		tmpfs	mode=1777	0	0
 tmpfs		/run		tmpfs	mode=0755,nosuid,nodev	0	0
 sysfs		/sys		sysfs	defaults	0	0