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

that are not plugins neither in the core

* use locking on the OpenGL context (using two APIs, but since agl needs the cgl headers included, should be ok i think).
  refs #668
  refs #915

  I'm not sure its fixed, but this seems like a logical way to solve it. Freaking apple with their 5 OpenGL APIs.