New user validation

[Brad Bishop]

Consider a DBus service for a moment that provides a user and group
management database (in the form of DBus objects).  Things like
usernames, group membership, etc.

Now consider an application using that service to create a new user.

Finally, consider a number of _other_ user facing applications
that need to abort the user creation process if some local validation
criteria fails.

Does anyone have any ideas on how a framework for doing such
validation might look?

For context: https://gerrit.openbmc-project.xyz/#/c/8440/

(Thank you Richard for pulling this together!)

thx - brad