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