put :decrypt and perhaps :decrypt-all in core

[geofft@mit.edu]

There's an external :decrypt command in Perl running around, e.g. in ​snippets and ​Nelson's Public, that allows you to decrypt a message if it wasn't decrypted when it arrived -- e.g., if you were missing the .crypt-table line, or if your tokens expired. This should really be in mainline.

Also, it should decrypt the message in the message list, instead of opening a popwin with the decrypted text.

Also, it would be nice to have something (either a command, or something automatically triggered) to do that for all messages that weren't decrypted when they arrived.