core: New encryption flag GPGME_ENCRYPT_THROW_KEYIDS.

* src/gpgme.h.in (GPGME_ENCRYPT_THROW_KEYIDS): New flag.
* src/engine-gpg.c (gpg_encrypt): Implement flag
(gpg_encrypt_sign): Implement flag.
* tests/run-encrypt.c (main): New option --throw-keyids.

It would be nice to also selectively hide recipients (that is gpg
--hidden-recipient) but our API does not ye allow this because it is
based on key objects. A possible way to implement that would be a API
to set processing flags into a key but this is complicated due to the
reference counting and thus the possibility that a key object is used
by different context.

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