Fix crash in res_pjsip on load if error occurs, and prevent unloading of res_pjsip...

Fix crash in res_pjsip on load if error occurs, and prevent unloading of res_pjsip and res_pjsip_session.

During load time in res_pjsip if an error occurred the operation would attempt to rollback all
operations done during load. This is not permitted by PJSIP as it will assert if the operation has
not been done. This fix changes the code so it will only rollback what has been initialized already.

Further changes also prevent res_pjsip and res_pjsip_session from being unloaded. This is due to
limitations within PJSIP itself. The library environment can only be changed to a certain extent
and does not provide the ability, currently, to deinitialize certain required functionality.

(closes issue ASTERISK-22474)
Reported by: Corey Farrell

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@399624 65c4cc65-6c06-0410-ace0-fbb531ad65f3