Defect in XSH _Exit

[Ben Harris <yyyyy@xxxxxxxxxx>]

@ page 85 line 3372-3374 section _Exit() editorial

Problem:
The SYNOPSIS lists _Exit() under <unistd.h>, whereas it should be under
<stdlib.h> (confirmed at XBD page 325 line 11650 section <stdlib.h> and
XSH page 305 line 9941 section exit()).

Action:
Replace the SYNOPSIS with:

#include <stdlib.h>

void _Exit(int status);

#include <unistd.h>

void _exit(int status);