isobusfs: treat interactive exit as control flow, not error

The cmd_exit() function used -EINTR to signal a clean exit from the
interactive mode. This is semantically incorrect because EINTR is an
errno value meant for system call interruptions, not application-level
control flow.

Stop using -EINTR for cmd_exit(). Introduce ISOBUSFS_CLI_RET_EXIT (a
positive return code) to distinguish control flow from actual errors.
Map this to exit code 0 in main(), keeping negative errno values for
real errors. This makes the exit path explicit and prevents confusion
with actual interrupted system calls.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>

isobusfs/isobusfs_cli.c

@@ -671,6 +671,10 @@ int main(int argc, char *argv[])
 
 	while (1) {
 		ret = isobusfs_cli_process_events_and_tasks(priv);
+		if (ret == ISOBUSFS_CLI_RET_EXIT) {
+			ret = 0;
+			break;
+		}
 		if (ret)
 			break;
 	}