init: change rootful detection condition (#909)

* Change rootful detection condition

`cat /run/host/etc/shadow > /dev/null` appears to always return true even when rootless.

Check to see if `/etc/shadow` is writable to determine access. The earlier commit for checking readability also always returned true.

* Update distrobox-init

distrobox-init

@@ -187,7 +187,9 @@ fi
 #
 # if /run/.nopasswd is present, let's treat the init as rootless, this is not
 # a good thing, users behold!
-if cat /run/host/etc/shadow > /dev/null && [ ! -e /run/.nopasswd ]; then
+if stat /run/host/etc/shadow &&
+	[ "$(stat -c "%u" /run/host/etc/shadow)" = "0" ] &&
+	[ ! -e /run/.nopasswd ]; then
 	rootful=1
 fi