res_pjsip: Fix startup/reload memory leak in config_auth.

res_pjsip: Fix startup/reload memory leak in config_auth.

An issue in config_auth.c:ast_sip_auth_digest_algorithms_vector_init() was
causing double allocations for the two supported_algorithms vectors to the
tune of 915 bytes.  The leak only happens on startup and when a reload is done
and doesn't get bigger with the number of auth objects defined.

* Pre-initialized the two vectors in config_auth:auth_alloc().
* Removed the allocations in ast_sip_auth_digest_algorithms_vector_init().
* Added a note to the doc for ast_sip_auth_digest_algorithms_vector_init()
  noting that the vector passed in should be initialized and empty.
* Simplified the create_artificial_auth() function in pjsip_distributor.
* Set the vector initialization count to 0 in config_global:global_apply().