I should have been more careful. The patch I posted before is not
needed.  I got confused, because the following program worked under
SunOS (while it should not). It prints 'Operation not permitted'
correctly on NetBSD...

#include <stdio.h>
#include <unistd.h>

int
main(argc, argv)
	int argc;
	char *argv[];
{
	if (argc == 1)
		return 2;

	if (setsid() == -1) {
		perror("setsid");
		exit(1);
	}

	execvp(argv[1], &argv[1]);
	perror("execvp");
	return 1;
}

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