sip: add preference to disable secure dialog checks for SIP/TLS

Some VoIP services, such as VoIP.ms, support SIP/TLS and SRTP, but
give the wrong schema in the INVITE's Contact header; specifically
"sip" instead of "sips." The relevant ticket for pjsip is as
follows:

https://trac.pjsip.org/repos/ticket/1735



This patch adds a preference to allow a user to disable the checks,
which allows them to answer incoming calls in this situation. By
default, the checks are enabled, and it will silently drop calls
which are not RFC-compliant.

Reviewed-by:  <mingrui.zhang@savoirfairelinux.com>
Change-Id: I401ef481ef29f7ae7bbc56025d2e8b461e850791