The keyfile backend forms paths like this:
prefix + group_name + '/' + keyname
If the prefix is '/apps/yelp/' and the group name is '/' then this means
that we end up with a key name of (for example):
'/apps/yelp/' + '/' + '/' + 'font-adjustment'
= '/apps/yelp///font-adjustment'
which is obviously not a valid key name.
This patch rejects group names starting or ending with '/' or containing
'//' and also rejects keys containing '/'. This should make it
impossible for invalid keys to be formed.
