gpg: Make --dry-run and show-only work for secret keys.

* g10/import.c (import_secret_one): Check for dry-run before
transferring keys.

The use of --dry-run or --import-option show-only had no effect when
importing a secret key and the public key already existed. If the
public key did not exist an error message inhibited the import of the
secret key.

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