Check stack segment correctly.
 [jmk] --rw-rw-r-- M 320859 glenda sys 15144 Oct 27 00:24 sys/src/9/port/sysproc.c
	/n/sourcesdump/2005/1027/plan9/sys/src/9/port/sysproc.c:754,761 - 
	/n/sourcesdump/2005/1028/plan9/sys/src/9/port/sysproc.c:754,763
	  	error(Ebadarg);
	  
	  found:
	- 	/* Check we are not detaching the current stack segment */
	- 	if((ulong)arg >= s->base && (ulong)arg < s->top) {
	+ 	/*
	+ 	 * Check we are not detaching the initial stack segment.
	+ 	 */
	+ 	if(s == up->seg[SSEG]){
	  		qunlock(&s->lk);
	  		error(Ebadarg);
	  	}