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
GnuPG-bug-id: 4392
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.

Signed-off-by: Werner Koch <wk@gnupg.org>

g10/pubkey-enc.c

@@ -117,7 +117,7 @@ get_session_key (ctrl_t ctrl, struct pubkey_enc_list *list, DEK *dek)
        * - On-card keys of an active card
        * - On-disk keys with protection
        * - On-card keys from cards which are not plugged it.  Here a
-       *   cancel-all button should stop aksing for other cards.
+       *   cancel-all button should stop asking for other cards.
        * Without any anonymous keys the sorting can be skipped.
        */
       for (k = list; k; k = k->next)