[Bug-tar] Problems extracting sparse files

[Jonathan Kamens]

Tar 1.14 doesn't extract sparse files properly when sending output
somewhere other than a plain file, e.g., to /dev/null or to a pipe.
This is because it tries to do things like seek and use ftruncate in
the output file.

It certainly needs to do the seeking and truncating when it's writing
to a real file, to preserve the sparseness of the file, but it also
needs to check if it's writing to a non-file, and if so, it needs to
actually write zeroes when it encounters sparse blocks rather than
trying to do operations that won't work on non-files.

In short, tar needs to be able to extract sparse files correctly even
when it isn't writing to a file.

  Jonathan Kamens