Fix the obscure at(1) permission problem.

Fix the obscure at(1) permission problem.

change_[er]uid() both use cratom(), however their consumers happened to cache
the original cred in a variable.  If the cred happened to be shared before
(for example due to an open(2)), a conditional lateron would still reference
the old cred instead of the new one, which was instanciated in change_[er]uid.

Fix this by returning the new cred from change_[er]uid and using this in
subsequent conditionals.

DragonFly-bug: http://bugs.dragonflybsd.org/issue509