Support PKCS 7 decryption

Description of the feature:

Add support for Enveloped data inside PKCS_7 containers. Major parts are already there thanks to encrypted PKCS_8

Applications that this feature may be relevant to:

All applications that would like to do S/MIME support with the help of GnuTLS. E.g. Mutt can benefit from using GnuTLS directly rather than calling openssl through external calls.

Is this feature implemented in other libraries (and which)

OpenSSL/LibreSSL/NSS.