Add support for DTLS-SRTP to res_rtp_asterisk and chan_sip.

Add support for DTLS-SRTP to res_rtp_asterisk and chan_sip.

As mentioned on the review for this, WebRTC has moved towards choosing
DTLS-SRTP as the mechanism for key exchange for SRTP. This commit adds
support for this but makes it available for normal SIP clients as well.

Testing has been done to ensure that this introduces no regressions with
existing behavior and also that it functions as expected.

Review: https://reviewboard.asterisk.org/r/2113/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@373229 65c4cc65-6c06-0410-ace0-fbb531ad65f3