agent: Let READKEY update the display-s/n of the Token entry.

* agent/findkey.c (agent_write_private_key): Factor file name
generation out to ...
(fname_from_keygrip): new.
(write_extended_private_key): Add and implement new arg MAYBE_UPDATE.
(agent_write_shadow_key): Ditto.

* agent/command.c (cmd_readkey): Update the shadow-key in card mode.
--

GnuPG-bug-id 6135

agent/learncard.c

@@ -413,8 +413,8 @@ agent_handle_learn (ctrl_t ctrl, int send, void *assuan_context, int force)
         char *dispserialno;
 
         agent_card_getattr (ctrl, "$DISPSERIALNO", &dispserialno);
-        rc = agent_write_shadow_key (grip, serialno, item->id, pubkey, force,
-                                     dispserialno);
+        rc = agent_write_shadow_key (0, grip, serialno, item->id, pubkey,
+                                     force, dispserialno);
         xfree (dispserialno);
       }
       xfree (pubkey);