Initial support for Secure Real-Time Protocol (RFC3711) - refs #321

Uses libgcrypt, as we have it in our deps/contrib already for GnuTLS.

This could be used in both "UDP"[1] access and access output plugins,
though they should really be called "RTP" instead nowadays.

Done:
 - AES(-128) Counter Mode key derivation and RTP en-/decryption
 - test vectors for AES-CM
 - NULL cipher (with the *_UNENCRYPTED flags) - untested

To do (missing mandatory features):
 - RTCP en-/decryption
 - HMAC-SHA1 authentication
 - replay attack protection

Also to probably do:
 - integrate with udp access and access output plugins
 - integrate with RTSP server (err, I won't do that myself)
 - support for Transform Carrying ROC for SRTP (RFC4771)
   so we can use it easily for multicast streaming