bug#40118: 27.0.90; Signing emails with gpg

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: stepnem@gmail.com,  40118@debbugs.gnu.org,  boudiccas@skimble.plus.com
> Date: Thu, 16 Apr 2020 11:44:51 +0200
> 
> In emacs-26, mml-secure-epg-sign could specify that a message should
> be signed with a key based on the senderʼs email address. If the
> variable governing that was nil, it was left to gpg to figure out the
> key to use. Normally that means gpg uses its default key.
> 
> In emacs-27, mml-secure-epg-sign now checks explicitly whether that
> variable is nil, and refuses to continue.

Why was this change in behavior made in Emacs 27?

> With an error message that in at least 50% of the cases points the
> user to the wrong user option. This is a regression from emacs-26.
> 
> Fixing the error message is easy. Iʼm proposing that by default the
> senderʼs email address is used to determine the key to use, since
> thatʼs what almost everyone will want. People who donʼt want that can
> control the behaviour by either adding keys to
> 'mml-secure-openpgp-signers' or by setting 'mm-sign-option' to
> 'guided.

I'd prefer to have a behavior that didn't require any changes, if
possible.  Thus the above question.  If having a compatible behavior
is impractical, then let's discuss what would the lesser evil.

Thanks.