please include --hidden-recipients in gpgme

[Daniel Kahn Gillmor]

gpgme_op_encrypt() and gpgme_op_encrypt_start() and gpgme_encrypt_sign()
and gpgme_op_encrypt_sign_start() all have an option:

    gpgme_key_t recp[]

for who the cleartext should be encrypted to.

But there appears to be no way to include a list of hidden recipients
(in the sense of gpg's --hidden-recipient option).

I'm not sure what the right way to introduce this would be -- perhaps we
need a second form of these functions, with an additional argument?

I considered the possibility of adding a flag that forces all recipients
to be hidden, but i think that --throw-keyids is too coarse of a hammer.
The most common use case is for e-mail, where there's no point in
obscuring the key IDs for recipients who are listed in the e-mail
headers (though it is important to be able to hide the keyIDs of the
recipients who are listed in Bcc during message composition).

Any suggestions on the best way to introduce this feature for future
users of gpgme?

           --dkg
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