On close, handle short writes from archive_write_callback

On close, handle short writes from archive_write_callback

The archive_write_callback passed to archive_write_open() is documented as:

 "each call to the write callback function should translate to a single write(2) system call.
  On success, the write callback should return the number of bytes actually written"

And in most places, the code repeatedly calls the write callback, but when flushing
the buffer at close, the write callback was called once, assuming it would write everything.
This could result in a truncated archive.

A test is added to test short writes in different code paths.