efibootmgr: -C isn't implemented

-C isn't implemented, so just errx out until it is. It's not listed in
the man page, but is parsed for compatibility with the Linux
efibootmgr(8) command.

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usr.sbin/efibootmgr/efibootmgr.c

@@ -226,6 +226,8 @@ parse_args(int argc, char *argv[])
 		case 'C':
 			opts.copy = true;
 			opts.cp_src = strtoul(optarg, NULL, 16);
+			errx(1, "Copy not implemented");
+			break;
 		case 'c':
 			opts.create = true;
 			break;