res_phoneprov: Refactor phoneprov to allow pluggable config providers

res_phoneprov: Refactor phoneprov to allow pluggable config providers

This patch makes res_phoneprov more modular so other modules (like pjsip)
can provide configuration information instead of res_phoneprov relying solely
on users.conf and sip.conf.  To accomplish this a new ast_phoneprov public API
is now exposed which allows config providers to register themselves, set
defaults (server profile, etc) and add user extensions.

* ast_phoneprov_provider_register registers the provider and provides callbacks
  for loading default settings and loading users.
* ast_phoneprov_provider_unregister clears the defaults and users.
* ast_phoneprov_add_extension should be called once for each user/extension
  by the provider's load_users callback to add them.
* ast_phoneprov_delete_extension deletes one extension.
* ast_phoneprov_delete_extensions deletes all extensions for the provider.

Tested-by: George Joseph
Review: https://reviewboard.asterisk.org/r/3970/
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Merged revisions 424963 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 424964 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@424965 65c4cc65-6c06-0410-ace0-fbb531ad65f3