Change DNA magic value to -1 to make UID 999 usable
Change user-add's uid & gid parameters from autofill to optional.
Change the DNA magic value to -1.
For old clients, which will still send 999 when they want DNA
assignment, translate the 999 to -1. This is done via a new
capability, optional_uid_params.
Tests included
https://fedorahosted.org/freeipa/ticket/2886