Add text that says that an interpreter cannot itself be an interpreter

file.  Inspired by Solaris manpage and the current NetBSD implementation
as observed with the scotty package and attempts to use the earlier shell
wrapper as an interpreter.

lib/libc/sys/execve.2

@@ -1,4 +1,4 @@
-.\"	$NetBSD: execve.2,v 1.18 2000/06/17 19:33:34 hubertf Exp $
+.\"	$NetBSD: execve.2,v 1.19 2000/07/05 12:42:09 he Exp $
 .\"
 .\" Copyright (c) 1980, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -93,6 +93,9 @@ become the subsequent arguments.  The zeroth argument, normally the name of the
 .\" was .Fn execve Ap d
 \fBexecve\fP'd
 file, is left unchanged.
+The interpreter named by
+.Em interpreter
+must not itself be an interpreter file.
 .Pp
 The argument
 .Fa argv