realtime: Fix ast_load_realtime_multientry handling

ast_load_realtime_multientry() returns an ast_config structure whose
ast_categorys are keyed with the empty strings. Several modules were
giving semantic meaning to the category names causing problems at
runtime.

* app_directory: Treated the category name as the mailbox name, and
  would fail to direct calls to the appropriate extension after an
  entry was chosen.

* app_queue: Queues, queue members, and queue rules were all affected
  and needed to be updated.

* pbx_realtime: Pattern matching would never succeed because the
  extension entered by the user was always compared to the empty
  string.

Change-Id: Ie7e44986344b0b76ea8f6ddb5879f5040c6ca8a7