sorcery: Make is_object_field_registered handle field names that are regexes.

As a result of https://reviewboard.asterisk.org/r/3305, res_sorcery_realtime
was tossing database fields that didn't have an exact match to a sorcery
registered field.  This broke the ability to use regexes as field names which
manifested itself as a failure of res_pjsip_phoneprov_provider which uses
this capability.  It also broke handling of fields that start with '@' in
realtime but I don't think anyone noticed.

This patch does the following...
* Modifies ast_sorcery_fields_register to pre-compile the name regex.
* Modifies ast_sorcery_is_object_field_registered to test the regex if it
  exists instead of doing an exact strcmp.
* Modifies res_pjsip_phoneprov_provider with a few tweaks to get it to work
  with realtime.

Tested-by: George Joseph

Review: https://reviewboard.asterisk.org/r/4185/
........

Merged revisions 428543 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@428544 f38db490-d61c-443f-a65b-d21fe96a405b