Write support for raw files

Hi,

I wanted to read and write raw files (foo.gz bar.bz2, ...) and while libarchive supports reading these files, writing without container was not supported. So Tim Kientzle told me to write a patch, here it goes.

I am aware that the code might not be ready for merging just yet, but my simple and stupid test worked fine, so I thought I might go ahead and publish it for review.

There's a number of things I was not sure about:

  * Can I just ignore functions I don't implement? It seems that the function to set options is indeed optional, but what about the others?
  * I had to tell GCC to ignore some parameters, because I just do not need them. Hope yo are fine with that.

If you find something, let me know and I'll fix it. Hope to get this change into libarchive soon!

P.S.: I saw that libarchive learned UU and Base64 recently, this can be used to implement the relevant Unix commands on top of the new filters.

regards,
Leonidas