Add to Address Book contact flags whether receiver supports Encrypted, Signed, or Unknown mail.

[achowe]

When posting to some mailing lists, eg. gnats-bugs@NetBSD.org, or people (like parents) for that matter might not be able to handle decrypting, verification, or stripping of public key attachment, see https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=57860

It would be useful add flags to such a contact to either enable/disable preferred sending options for encryption, digital signature, or unknown (default).

[Jon]

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