testing the right thing lower down (and so that future code doesn't
  always have to remember to save it off and restore it).
- Improved a chunk of link_dest code.
- Handle the removal of a non-regular file without so much duplicated
  code.

reading.

the batchfile and read_stream_flags() to read and twiddle the same
flags.  This ensures that the batchfile reading doesn't get confused
about what data to expect from the socket.

it to ensure that rename() didn't lie to us (which it does if you try to
rename() a file over another link to the same file).

64 bits.  The code now only complains if int64 is really too short.

for keep_partial handling.

to set_perms() if ok_to_set_time wasn't specified.

MSG_REDO so that the output comes out in the right order.

know if we retained it or discarded it.  Especially useful for batch-
reading mode where the old code could look like it did the update when
it really silently failed.

implies the latter.

only process the updates from the batch file when the generator has
indicated that it is ready (which ensures that all the necessary
dirs have been created).

- Avoid calling local_child().
- Create a pipe that lets the generator send us index values.
- Set batch_gen_fd for the receiver to read the pipe.

to the f_out pipe before we return from recv_generator().  This gives
the receiver something to sync with so that it doesn't rush ahead of
us (which could be bad if we didn't have a chance to create the
destination dirs yet).

call to map_file() until the block size was known).

  call to map_file() until the block size was known).
- Got rid of the setting of max_map_size.

  new def_window_size member variable.
- Got rid of max_map_size global (we use def_window_size now).