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.