Move atexit into its own file.
 [rsc] --rw-rw-r-- M 28433 rsc sys 220 Sep 19 06:47 sys/src/ape/lib/ap/stdio/atexit.c
 [rsc] --rw-rw-r-- M 28433 glenda sys 321 Sep 19 06:47 sys/src/ape/lib/ap/stdio/exit.c
	/n/sourcesdump/2005/0919/plan9/sys/src/ape/lib/ap/stdio/exit.c:15,27 - 
	/n/sourcesdump/2005/0920/plan9/sys/src/ape/lib/ap/stdio/exit.c:15,17
	  	_doatexits();
	  	_exit(status);
	  }
	- atexit(int (*f)(void))
	- {
	- 	int i;
	- 	for(i=0; i<NONEXIT; i++)
	- 		if(!_atexitfns[i]){
	- 			_atexitfns[i] = f;
	- 			return(0);
	- 		}
	- 	return(1);
	- }
 [rsc] --rw-rw-r-- M 28433 glenda sys 841 Sep 19 06:47 sys/src/ape/lib/ap/stdio/mkfile
	/n/sourcesdump/2005/0919/plan9/sys/src/ape/lib/ap/stdio/mkfile:7,12 - 
	/n/sourcesdump/2005/0920/plan9/sys/src/ape/lib/ap/stdio/mkfile:7,13
	  	_dtoa.$O\
	  	_fconv.$O\
	  	clearerr.$O\
	+ 	atexit.$O\
	  	exit.$O\
	  	fclose.$O\
	  	fdopen.$O\