gpg: Avoid importing secret keys if the keyblock is not valid.

* g10/keydb.h (struct kbnode_struct): Replace unused field RECNO by
new field TAG.
* g10/kbnode.c (alloc_node): Change accordingly.
* g10/import.c (import_one): Add arg r_valid.
(sec_to_pub_keyblock): Set tags.
(resync_sec_with_pub_keyblock): New.
(import_secret_one): Change return code to gpg_error_t.   Return an
error code if sec_to_pub_keyblock failed.  Resync secret keyblock.

When importing an invalid secret key ring for example without key
binding signatures or no UIDs, gpg used to let gpg-agent store the
secret keys anyway. This is clearly a bug because the diagnostics
before claimed that for example the subkeys have been skipped.
Importing the secret key parameters then anyway is surprising in
particular because a gpg -k does not show the key. After importing
the public key the secret keys suddenly showed up.

This changes the behaviour of
to me more consistent but is not a solution to the actual bug.

Caution: The ecc.scm test now fails because two of the sample keys

don't have binding signatures.

• GnuPG-bug-id: T4392
• Signed-off-by: Werner Koch <wk@gnupg.org>