res_pjsip_sdp_rtp: Add support for optimistic SRTP.

Optimistic SRTP is the ability to enable SRTP but not have it be
a fatal requirement. If SRTP can be used it will be, if not it won't be.
This gives you a better chance of using it without having your sessions
fail when it can't be.

Encrypt all the things!

Review: https://reviewboard.asterisk.org/r/3992/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@428222 f38db490-d61c-443f-a65b-d21fe96a405b