Use srtp_shutdown when available

This allows the SRTP library to be shut down properly when the
functionality is offered by libsrtp.

Review: https://reviewboard.asterisk.org/r/2538/
(closes issue ASTERISK-21719)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@388768 f38db490-d61c-443f-a65b-d21fe96a405b