Generate a ludus UID from the linux UID that is compatible with ludus restrictions

Ludus restricts the format of a UID to ^[A-Za-z0-9]{1,20}$ which is more restrictive than linux.

The UID can thus not be directly the one returned by PAM but must be used to generate a unique UID that is compatible with ludus.

A first implementation ID would be to:

• strip all unwanted character
• keep the 10 first ones
• fill with the 10 first character of a sha256 of the original userid

That should generate a uid that is unique