This is not very useful at the moment, but will be needed for proper
replay attack protection

It has to be at least 64, and it is too complicated to implement a bigger
value, so it's not going to be configurable

Revert [19273] because these asserts are meant to catch serious programming errors. Thanks zorglub for enlightning me.

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