Zencode for plain symmetric AES GCM encryption

[jaromil]

Currently the AES GCM transformation is bound to asymmetric ECDH sessions.

We should have simple zencode statements for AES GCM encoding with all parameters exposed (mostly IV and header).

This implementation should use have and mayhave to retrieve needed parameters to be explicit in error messages about which ones are missing. It may need to use the hidden CACHE, in such a case the zencode_fsp implementation may serve as example.