Buffer warning/error outputs to avoid interlacing in parallel mk.
 [rsc] --rw-rw-r-- M 323293 glenda sys 12071 Nov  4 08:20 sys/src/cmd/cc/cc.h
	/n/sourcesdump/2005/1104/plan9/sys/src/cmd/cc/cc.h:449,454 - 
	/n/sourcesdump/2005/1105/plan9/sys/src/cmd/cc/cc.h:449,455
	  EXTERN	Node*	nodproto;
	  EXTERN	Node*	nodcast;
	  EXTERN	Biobuf	outbuf;
	+ EXTERN	Biobuf	diagbuf;
	  EXTERN	char*	outfile;
	  EXTERN	char*	pathname;
	  EXTERN	int	peekc;
 [rsc] --rw-rw-r-- M 323293 glenda sys 34138 Nov  4 08:20 sys/src/cmd/cc/sub.c
	/n/sourcesdump/2005/1104/plan9/sys/src/cmd/cc/sub.c:1159,1168 - 
	/n/sourcesdump/2005/1105/plan9/sys/src/cmd/cc/sub.c:1159,1170
	  	va_start(arg, fmt);
	  	vseprint(buf, buf+sizeof(buf), fmt, arg);
	  	va_end(arg);
	- 	print("%L %s\n", (n==Z)? nearln: n->lineno, buf);
	+ 	Bprint(&diagbuf, "%L %s\n", (n==Z)? nearln: n->lineno, buf);
	  
	- 	if(debug['X'])
	+ 	if(debug['X']){
	+ 		Bflush(&diagbuf);
	  		abort();
	+ 	}
	  	if(n != Z)
	  	if(debug['v'])
	  		prtree(n, "diagnostic");
	/n/sourcesdump/2005/1104/plan9/sys/src/cmd/cc/sub.c:1169,1175 - 
	/n/sourcesdump/2005/1105/plan9/sys/src/cmd/cc/sub.c:1171,1177
	  
	  	nerrors++;
	  	if(nerrors > 10) {
	- 		print("too many errors\n");
	+ 		Bprint(&diagbuf, "too many errors\n");
	  		errorexit();
	  	}
	  }
	/n/sourcesdump/2005/1104/plan9/sys/src/cmd/cc/sub.c:1181,1191 - 
	/n/sourcesdump/2005/1105/plan9/sys/src/cmd/cc/sub.c:1183,1193
	  	va_list arg;
	  
	  	if(debug['w']) {
	- 		print("warning: ");
	+ 		Bprint(&diagbuf, "warning: ");
	  		va_start(arg, fmt);
	  		vseprint(buf, buf+sizeof(buf), fmt, arg);
	  		va_end(arg);
	- 		print("%L %s\n", (n==Z)? nearln: n->lineno, buf);
	+ 		Bprint(&diagbuf, "%L %s\n", (n==Z)? nearln: n->lineno, buf);
	  
	  		if(n != Z)
	  		if(debug['v'])
	/n/sourcesdump/2005/1104/plan9/sys/src/cmd/cc/sub.c:1209,1218 - 
	/n/sourcesdump/2005/1105/plan9/sys/src/cmd/cc/sub.c:1211,1220
	  	va_start(arg, fmt);
	  	vseprint(buf, buf+sizeof(buf), fmt, arg);
	  	va_end(arg);
	- 	print("%L %s\n", lineno, buf);
	+ 	Bprint(&diagbuf, "%L %s\n", lineno, buf);
	  	nerrors++;
	  	if(nerrors > 10) {
	- 		print("too many errors\n");
	+ 		Bprint(&diagbuf, "too many errors\n");
	  		errorexit();
	  	}
	  }
	/n/sourcesdump/2005/1104/plan9/sys/src/cmd/cc/sub.c:1226,1235 - 
	/n/sourcesdump/2005/1105/plan9/sys/src/cmd/cc/sub.c:1228,1239
	  	va_start(arg, fmt);
	  	vseprint(buf, buf+sizeof(buf), fmt, arg);
	  	va_end(arg);
	- 	print("%L %s\n", (n==Z)? nearln: n->lineno, buf);
	+ 	Bprint(&diagbuf, "%L %s\n", (n==Z)? nearln: n->lineno, buf);
	  
	- 	if(debug['X'])
	+ 	if(debug['X']){
	+ 		Bflush(&diagbuf);
	  		abort();
	+ 	}
	  	if(n != Z)
	  	if(debug['v'])
	  		prtree(n, "diagnostic");