auth: Get rid of unneeded CONF_CHECK macro

This was used to ensure that config entries appeared "in order", when
the following two configurations were the same:

      class all {          class all
         timeout 300       class.timeout 300
      }

A user using the second config format could have easily written

      class.timeout 300
      class all

which would have caused a segfault in the class.timeout handler, as no
class had been created yet.
conf: Make u_conf_traverse perform iteration