* exec.cc (execvp): Call find_exec with FE_NNF flag to enforce

	* exec.cc (execvp): Call find_exec with FE_NNF flag to enforce
	a NULL return when executable isn't found in $PATH.  Convert NULL
	to "".
	(execvpe): Ditto.
	* spawn.cc (spawn_guts): Return with EFAULT if prog_arg is NULL.
	Return with ENOENT if prog_arg is empty string.  Add a comment.