Making readdir return '.' and '..' is trivially easy, as all the
required information to fake '.' and '..' entries are present.
The lack of '.' and '..' entries hasn't caused any problems despite
cramfs/squashfs being used for a large number of years. I'm inclined
to believe any application that _relies_ on seeing '.' and '..'
returned by readdir is broken. This situation is easily fixed within
the application rather than forcing the filesystem to unnecessarily
fake '.' and '..' entries which are never used.