sorcery: Create AST_SORCERY dialplan function.

This patch creates the AST_SORCERY dialplan function which allows someone to
retrieve any value from a sorcery-based config file.  It's similar to 
AST_CONFIG.

The creation of the function itself was fairly straightforward but it required
changes to the underlying sorcery infrastructure that rippled into individual
sorcery objects.  The changes stemmed from inconsistencies in how sorcery
created ast_variable objectsets from sorcery objects and the inconsistency
in how individual objects used that feature especially when it came to
parameters that can be specified multiple times like contact in aor and match
in identify.  You can read more here...
http://lists.digium.com/pipermail/asterisk-dev/2014-February/065202.html

So, what this patch does, besides actually creating the AST_SORCERY function,
is the following...

* Creates ast_variable_list_append which is a helper to append one ast_variable
  list to another.
* Modifies the ast_sorcery_object_field_register functions to accept the
  already-defined sorcery_fields_handler callback.
* Modifies ast_sorcery_objectset_create to accept a parameter indicating return
  type preference...a single ast_variable with all values concatenated or an
  ast_variable list with multiple entries.  Also fixed a few bugs.
* Modifies individual sorcery object implementations to use the new function
  definition of the ast_sorcery_object_field_register functions.
* Modifies location.c and res_pjsip_endpoint_identifier_ip.c to implement
  sorcery_fields_handler handlers so they return multiple occurrences as an
  ast_variable_list.
* Added a whole bunch of tests to test_sorcery.

(closes issue ASTERISK-22537)
Review: http://reviewboard.asterisk.org/r/3254/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@410042 f38db490-d61c-443f-a65b-d21fe96a405b