gnus-cloud native GnuTLS AEAD support

[Ted Zlatanov]

I've pushed scratch/tzz/gnus-cloud-aead with a proposed implementation
to use the native GnuTLS AEAD symmetric ciphers for gnus-cloud storage.
While this specifically is for IMAP articles, the implementation can
encrypt/decrypt any unibyte text data using AEAD ciphers.

Reasons to implement it this way:

* no dependency on external tools
* low risk: optional storage method for non-critical data

TODO:

* move the new code out to a library, either in core or in the GNU ELPA
* document, factor some things out
* add a checker function to read-passwd for min/max etc. requirements

It worked well in my testing. Comments and suggestions welcome.

Ted