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