Sections Exist in pjsip.conf

This patch modifies the current loading strategy of the pjsip configuration. If
duplicate sections (e.g. sections containing the same [id/type]) are defined in
[pjsip.conf], the loader will consider the configuration for the given type as
invalid when the duplicate section is encountered. The entire configuration
(including what was previously loaded) for the duplicate [id/type] sections
will be rejected and destroyed, an error message is logged and the load
processing for the given stops.

ASTERISK-24996
Reported By: Ashley Sanders

Change-Id: I35090ca4cd40f1f34881dfe701a329145c347aef