core: New GPGME_DATA_ENCODING_MIME.

* src/gpgme.h.in (GPGME_DATA_ENCODING_MIME): New.
* src/data.c (gpgme_data_set_encoding): Adjust check.
* src/engine-gpg.c (have_gpg_version): New.
(gpg_encrypt, gpg_encrypt_sign): Pass flag '--mimemode'.
(gpg_sign): Ditto.
* lang/cpp/src/data.h (GpgME): Add MimeEncoding.
* lang/cpp/src/data.cpp (encoding, setEncoding): Support MimeEncoding.
* src/gpgme-tool.c (server_data_encoding): Add flag --mime.

This feature allows an application to declare that the encrypted or
signed data is a valid MIME part.

What is missing is a way to return that information to the application
after decryption/verification. This can be done by setting the
encoding of the output data object; however this requires some
internal additions to our processing model.

• Signed-off-by: Werner Koch <wk@gnupg.org>