Dpkg::Backend::GnuPG: Ensure future signing interop with gpg --openpgp

GnuPG upstream has decided to get out of the standardizing process for
OpenPGP, and instead is trying to push its own proprietary fork based on
an old draft that did not have consensus within the IETF working group.

This is going to be a source of interoperability problems, but we can
mitigate them somewhat when creating signatures by requiring compliance
with the OpenPGP RFC, even if it's going to be locked into an old version,
as later ones are not planned to get implemented. More so, given that the
latest releases of GnuPG have been switched to default to the proprietary
draft.

scripts/Dpkg/OpenPGP/Backend/GnuPG.pm

@@ -257,6 +257,7 @@ sub inline_sign {
     my @exec = ($self->{cmd});
     push @exec, _gpg_options_weak_digests();
     push @exec, qw(--utf8-strings --textmode --armor);
+    push @exec, '--openpgp';
     if ($key->type eq 'keyfile') {
         # Promote the keyfile keyhandle to a keystore, this way we share the
         # same gpg-agent and can get any password cached.