res_pjsip.c: Fix ident_to_str() and refactor ident_handler().

* Extracted sip_endpoint_identifier_type2str() and
sip_endpoint_identifier_str2type() to simplify the calling functions.

* Fixed pjsip_configuration.c:ident_to_str() building the endpoint's
identify_by value string.

Change-Id: Ide876768a8d5d828b12052e2a75008b0563fc509