gpg: Take care to use pubring.kbx if it has ever been used.

* kbx/keybox-defs.h (struct keybox_handle): Add field for_openpgp.
* kbx/keybox-file.c (_keybox_write_header_blob): Set openpgp header
flag.
* kbx/keybox-blob.c (_keybox_update_header_blob): Add arg for_openpgp
and set header flag.
* kbx/keybox-init.c (keybox_new): Rename to do_keybox_new, make static
and add arg for_openpgp.
(keybox_new_openpgp, keybox_new_x509): New.  Use them instead of the
former keybox_new.
* kbx/keybox-update.c (blob_filecopy): Add arg for_openpgp and set the
openpgp header flags.

* g10/keydb.c (rt_from_file): New.  Factored out and extended from
keydb_add_resource.
(keydb_add_resource): Switch to the kbx file if it has the openpgp
flag set.

* kbx/keybox-dump.c (dump_header_blob): Print header flags.
--

The problem was reported by dkg on gnupg-devel (2014-10-07):

  I just discovered a new problem, though, which will affect people on
  systems that have gpg and gpg2 coinstalled:

   0) create a new keyring with gpg2, and use it exclusively with gpg2
  for a while.
   1) somehow (accidentally?) use gpg (1.4.x) again -- this creates
  ~/.gnupg/pubring.gpg
   2) future runs of gpg2 now only look at pubring.gpg and ignore
  pubring.kbx -- the keys you had accumulated in the keybox are no
  longer listed in the output of gpg2 --list-keys

Note that gpgsm has always used pubring.kbx and thus this file might
already be there but without gpg ever inserted a key.  The new flag in
the KBX header gives us an indication whether a KBX file has ever been
written by gpg >= 2.1.  If that is the case we will use it instead of
the default pubring.gpg.

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

kbx/keybox-blob.c

@@ -42,8 +42,9 @@
    - u32  Length of this blob
    - byte Blob type (1)
    - byte Version number (1)
-   - byte RFU
-   - byte RFU
+   - u16  Header flags
+          bit 0 - RFU
+          bit 1 - Is being or has been used for OpenPGP blobs
    - b4   Magic 'KBXf'
    - u32  RFU
    - u32  file_created_at
@@ -1028,7 +1029,7 @@ _keybox_get_blob_fileoffset (KEYBOXBLOB blob)
 
 
 void
-_keybox_update_header_blob (KEYBOXBLOB blob)
+_keybox_update_header_blob (KEYBOXBLOB blob, int for_openpgp)
 {
   if (blob->bloblen >= 32 && blob->blob[4] == BLOBTYPE_HEADER)
     {
@@ -1039,5 +1040,8 @@ _keybox_update_header_blob (KEYBOXBLOB blob)
       blob->blob[20+1] = (val >> 16);
       blob->blob[20+2] = (val >>  8);
       blob->blob[20+3] = (val      );
+
+      if (for_openpgp)
+        blob->blob[7] |= 0x02;  /* OpenPGP data may be available.  */
     }
 }