Make sure selinuxusemap behaves consistently to HBAC rule
Both selinuxusermap-add and selinuxusermap-mod commands now behave
consistently in not allowing user/host category or user/host members
and HBAC rule being set at the same time. Also adds a bunch of unit
tests that check this behaviour.
https://fedorahosted.org/freeipa/ticket/2983