res_pjsip: Fix CLI tab completion issues

res_pjsip: Fix CLI tab completion issues

This fixes several issues with the new res_pjsip CLI tab completion
such as output of headers during tab completion and being able to
tab-complete more items than the code actually handled (further items
would simply be ignored).

(closes issue ASTERISK-23081)
Review: https://reviewboard.asterisk.org/r/3115/
Reported by: xrobau
........

Merged revisions 405338 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@405339 65c4cc65-6c06-0410-ace0-fbb531ad65f3