Create a relative symlink for log files.

This matches what C++ does in practice, and permits this code to work
if -log_dir is a relative path.

glog_file.go

diff --git a/glog_file.go b/glog_file.go
index 8dcb2bc..65075d2 100644
--- a/glog_file.go
+++ b/glog_file.go
@@ -114,8 +114,8 @@
 		f, err := os.Create(fname)
 		if err == nil {
 			symlink := filepath.Join(dir, link)
-			os.Remove(symlink)         // ignore err
-			os.Symlink(fname, symlink) // ignore err
+			os.Remove(symlink)        // ignore err
+			os.Symlink(name, symlink) // ignore err
 			return f, fname, nil
 		}
 		lastErr = err