Revamp the different su_ud_* subtypes

For reap(), su_call() now only gets the SV cb and not the whole su_ud_reap
structure. This lets us free the ud in su_pop(), and likewise for
su_localize(). As for the uid feature, it gets its own su_ud_uid structure
instead of hijacking su_ud_reap, so that su_uid_bump() is no longer needed.