Allow to specify an enumlist inside an enum.

It was not possible to use an enumlist inside an enum:
<enumlist>
   <enum name="aa">
      <enumlist>
         ...
      </enumlist>
   </enum>
</enumlist>
Now we will be able to insert as many levels as we want.

(closes issue #15112)
Reported by: lmadsen



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@194635 65c4cc65-6c06-0410-ace0-fbb531ad65f3