Allow principals to refer to nonexistent policies

Allow principals to refer to nonexistent policies

Stop using and maintaining the policy_refcnt field, and do not try to
prevent deletion of a policy which is still referenced by principals.
Instead, allow principals to refer to policy names which do not exist
as policy objects; treat those principals as having no associated
policy.

In the kadmin client, warn if addprinc or modprinc tries to reference
a policy which doesn't exist, since the server will no longer error
out in this case.

ticket: 7385