Avoid dereferencing nil on bad fid.
Return partial Rwalks instead of Rerrors.
Post service ORCLOSE so it goes away when ramfs dies.
 [sys] --rwxrwxr-x M 259529 glenda sys 89789 Sep  6 23:11 386/bin/ramfs
	/sys/src/cmd/ramfs.c:io
	/sys/src/cmd/ramfs.c:main
	/sys/src/cmd/ramfs.c:rwalk
 [rsc] --rw-rw-r-- M 259529 glenda sys 15927 Sep  6 07:24 sys/src/cmd/ramfs.c
	/n/sourcesdump/2005/0906/plan9/sys/src/cmd/ramfs.c:88,93 - 
	/n/sourcesdump/2005/0907/plan9/sys/src/cmd/ramfs.c:88,109
	  	*rread(Fid*), *rwrite(Fid*), *rclunk(Fid*),
	  	*rremove(Fid*), *rstat(Fid*), *rwstat(Fid*);
	  
	+ int needfid[] = {
	+ 	[Tversion] 0,
	+ 	[Tflush] 0,
	+ 	[Tauth] 0,
	+ 	[Tattach] 0,
	+ 	[Twalk] 1,
	+ 	[Topen] 1,
	+ 	[Tcreate] 1,
	+ 	[Tread] 1,
	+ 	[Twrite] 1,
	+ 	[Tclunk] 1,
	+ 	[Tremove] 1,
	+ 	[Tstat] 1,
	+ 	[Twstat] 1,
	+ };
	+ 
	  char 	*(*fcalls[])(Fid*) = {
	  	[Tversion]	rversion,
	  	[Tflush]	rflush,
	/n/sourcesdump/2005/0906/plan9/sys/src/cmd/ramfs.c:177,183 - 
	/n/sourcesdump/2005/0907/plan9/sys/src/cmd/ramfs.c:193,199
	  		if(defmnt == 0){
	  			char buf[64];
	  			snprint(buf, sizeof buf, "#s/%s", service);
	- 			fd = create(buf, OWRITE, 0666);
	+ 			fd = create(buf, OWRITE|ORCLOSE, 0666);
	  			if(fd < 0)
	  				error("create failed");
	  			sprint(buf, "%d", p[1]);
	/n/sourcesdump/2005/0906/plan9/sys/src/cmd/ramfs.c:351,356 - 
	/n/sourcesdump/2005/0907/plan9/sys/src/cmd/ramfs.c:367,374
	  		f->busy = 0;
	  		f->ram = nil;
	  	}
	+ 	if(rhdr.nwqid > 0)
	+ 		err = nil;	/* didn't get everything in 9P2000 right! */
	  	if(rhdr.nwqid == thdr.nwname)	/* update the fid after a successful walk */
	  		f->ram = fram;
	  	return err;
	/n/sourcesdump/2005/0906/plan9/sys/src/cmd/ramfs.c:747,752 - 
	/n/sourcesdump/2005/0907/plan9/sys/src/cmd/ramfs.c:765,771
	  {
	  	char *err, buf[40];
	  	int n, pid, ctl;
	+ 	Fid *fid;
	  
	  	pid = getpid();
	  	if(private){
	/n/sourcesdump/2005/0906/plan9/sys/src/cmd/ramfs.c:787,796 - 
	/n/sourcesdump/2005/0907/plan9/sys/src/cmd/ramfs.c:806,817
	  		if(debug)
	  			fprint(2, "ramfs %d:<-%F\n", pid, &thdr);
	  
	- 		if(!fcalls[thdr.type])
	+ 		if(thdr.type<0 || thdr.type>=nelem(fcalls) || !fcalls[thdr.type])
	  			err = "bad fcall type";
	+ 		else if(((fid=newfid(thdr.fid))==nil || !fid->ram) && needfid[thdr.type])
	+ 			err = "fid not in use";
	  		else
	- 			err = (*fcalls[thdr.type])(newfid(thdr.fid));
	+ 			err = (*fcalls[thdr.type])(fid);
	  		if(err){
	  			rhdr.type = Rerror;
	  			rhdr.ename = err;
 [sys] --rwxrwxr-x M 259529 glenda sys 89789 Sep  6 23:11 386/bin/ramfs
	/sys/src/cmd/ramfs.c:emalloc
	/sys/src/cmd/ramfs.c:erealloc
	/sys/src/cmd/ramfs.c:error
	/sys/src/cmd/ramfs.c:estrdup
	/sys/src/cmd/ramfs.c:io
	/sys/src/cmd/ramfs.c:main
	/sys/src/cmd/ramfs.c:rwalk
	/sys/src/cmd/ramfs.c:usage
	/sys/src/libc/port/rune.c:chartorune