devfs: explicitly mount /dev/shm with mode=1777

The current code relies on the tmpfs mount defaulting to 1777
permissions.  If it doesn't, things break badly.

This can come up when tmpfs is disabled in the kernel and ramfs
is being used instead (the kernel will alias tmpfs to ramfs).
The default permissions for ramfs is 0755.

init.d/devfs.in

@@ -71,7 +71,7 @@ seed_dev()
 	for x in \
 		"mqueue /dev/mqueue 1777 ,nodev mqueue" \
 		"devpts /dev/pts 0755 ,gid=5,mode=0620 devpts" \
-		"tmpfs /dev/shm 1777 ,nodev shm" \
+		"tmpfs /dev/shm 1777 ,nodev,mode=1777 shm" \
 	; do
 		set -- $x
 		grep -Eq "[[:space:]]+$1$" /proc/filesystems || continue