simplify and regularize cfg_* functions

simplify and regularize cfg_* functions

- several functions that can no longer fail have been changed to
  type void, and unnecessary 'cleanup' sections were removed
- renamed cfg_create_obj() to cfg_obj_create(), and cfg_create_tuple()
  to cfg_tuple_create(), to match typical nomenclature.
- fixed a memory leak bug, in which an element could be removed
  from a list in delete_zoneconf() without being freed. this has
  been addressed by adding a cfg_list_unlink() function.
  list elements are now allocated based on the list they will
  be stored in, using the same mctx.