Implement aes_ctr128 and prefer it over RC4.

Implement aes_ctr128 and prefer it over RC4.

This adds the aes_ctr128 stream cipher to para_server, para_client
and para_audiod. para_server now announces this cipher as a supported
feature and para_client requests an aes_ctr128-encrypted session
if the server supports it. If one or both sides don't understand
the new feature, the RC4 fallback applies. So this change is
backwards-compatible.

Both gcrypt and openssl support aes_ctr128, and the two implementations
are compatible to each other: a para_client linked against openssl can
talk to a para_server which was linked against libgcrypt and vice versa.

This patch also updates the documentation to talk about stream
ciphers and session keys rather than to mention RC4 explicitly. A
short section on AES is added which also explains how counter mode
turns a block cipher into a stream cipher.