cmd/dist: force line-buffering stdout/stderr on Unix

If stdout and stderr are indeed the same file (not a tty), which is
        often the case, fully-buffered stdout will make it harder to see
        progresses, for example, ./make.bash 2>&1 | tee log

R=r, rsc
CC=golang-dev
https://golang.org/cl/5700070

src/cmd/dist/unix.c

@@ -641,6 +641,9 @@ main(int argc, char **argv)
 	Buf b;
 	struct utsname u;
 
+	setvbuf(stdout, nil, _IOLBF, 0);
+	setvbuf(stderr, nil, _IOLBF, 0);
+
 	binit(&b);
 	
 	slash = "/";