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

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

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@429739 65c4cc65-6c06-0410-ace0-fbb531ad65f3