pgcrypto: Add support for CFB mode in AES encryption

pgcrypto: Add support for CFB mode in AES encryption

Cipher Feedback Mode, CFB, is a self-synchronizing stream cipher which
is very similar to CBC performed in reverse. Since OpenSSL supports it,
we can easily plug it into the existing cipher selection code without
any need for infrastructure changes.

This patch was simultaneously submitted by Umar Hayat and Vladyslav
Nebozhyn, the latter whom suggested the feauture. The committed patch
is Umar's version.

Author: Umar Hayat <postgresql.wizard@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/CAPBGcbxo9ASzq14VTpQp3mnUJ5omdgTWUJOvWV0L6nNigWE5jw@mail.gmail.com