sm: Always create a keybox header when creating a new keybox.
* sm/keydb.c (maybe_create_keybox): Create the header blob.
This is required so that g10/keydb.c can properly detect that a keybox
file is actually there. Just writing a 0 zero length keybox file is
not sufficient because a file with that name may also be an old-style
OpenPGP keyring.
- GnuPG-bug-id: T2275
- Signed-off-by: Werner Koch <wk@gnupg.org>