agent: IMPORT_KEY with --force option fix.

* agent/cvt-openpgp.c (convert_from_openpgp_main): Add an option not
to check existing key.
(convert_from_openpgp): Ditto.
(convert_from_openpgp_native): Call convert_from_openpgp_main with
dontcare_exist=0.
* agent/command.c (cmd_import_key): Call with dontcare_exist=force.

agent/cvt-openpgp.h

@@ -20,6 +20,7 @@
 #define GNUPG_AGENT_CVT_OPENPGP_H
 
 gpg_error_t convert_from_openpgp (ctrl_t ctrl, gcry_sexp_t s_pgp,
+                                  int dontcare_exist,
                                   unsigned char *grip, const char *prompt,
                                   const char *cache_nonce,
                                   unsigned char **r_key, char **r_passphrase);