Ensure the correct value is returned for CHANNEL(pjsip, secure)

Prior to this patch, we were using the PJSIP dialog's secure flag
to determine if a secure transport was being used. Unfortunately,
the dialog's secure flag was only set if a SIPS URI were in use,
as required by RFC 3261 sections 12.1.1 and 12.1.2. What we're interested
in is not dialog security, but transport security. This code change
switches to a model where we use the dialog's target URI to determine
what transport would be used to communicate, and then check if that
transport is secure.

AST-1450 #close
Reported by John Bigelow

Review: https://reviewboard.asterisk.org/r/4277
........

Merged revisions 429739 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@429740 f38db490-d61c-443f-a65b-d21fe96a405b