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>

tests/openpgp/samplekeys/README

@@ -29,3 +29,5 @@ Notes:
   such a file is created which is then directly followed by a separate
   armored public key block.  To create such a sample concatenate
   pgp-desktop-skr.asc and E657FB607BB4F21C90BB6651BC067AF28BC90111.asc
+- ecc-sample-2-sec.asc and ecc-sample-3-sec.asc do not have and
+  binding signatures either.  ecc-sample-1-sec.asc has them, though.