+ move the enum declaration for output formats near the head

+ move the enum declaration for output formats near the head
  of the file, so it can be used from more places;
+ make the declaration of contenttype[] more robust;
+ remove the wrappers around __xml_translate(), since they were
  used only in one place, and rename to xml_translate().
  This allows for a bit of simplifications.
+ document the output produced by the above function.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@45485 65c4cc65-6c06-0410-ace0-fbb531ad65f3