src/pxar/sequential_decoder.rs: remove callback from new()

And use an extra functzion  set_callback() to configure that.

Also rewrite pxar/fuse.rs and implement a generic Session (will get
further cleanups with next patches).

src/client/pxar_decode_writer.rs

@@ -31,7 +31,8 @@ impl PxarDecodeWriter {
         
         let child = thread::spawn(move|| {
             let mut reader = unsafe { std::fs::File::from_raw_fd(rx) };
-            let mut decoder = pxar::SequentialDecoder::new(&mut reader, pxar::flags::DEFAULT, |path| {
+            let mut decoder = pxar::SequentialDecoder::new(&mut reader, pxar::flags::DEFAULT);
+            decoder.set_callback(move |path| {
                 if verbose {
                     println!("{:?}", path);
                 }