cpp: Use gpgme_op_decrypt_ex and add new flags.
* lang/cpp/src/context.cpp: New decrypt and decryptVerify functions that take flags as arguments. Use new variants in old functions. (Context::setDecryptionFlags): New helper. (Context::Private::Private): Initialize new member. * lang/cpp/src/context_p.h (Context::Private::decryptFlags): New. * lang/cpp/src/context.h (Context::DecryptFlags): New enum. (Context::EncryptionFlags): Extend for EncryptWrap.
The setDecryptionFlags provides a generic way to set decryption
flags for the whole context. This allows existing code to just
keep using the old functions and modify the decryption behavior
in a central place.