Pass used compression to caller when decoding deb file.

[ebikt]

Packages built on modern debians are not installable on old debians, due to used compression (data.tar.xz). I need to forbid such packages to get to repositories for old systems. It would be nice to pass filename of control and data tar to caller (e.g. by extending 'deb' struct).

[paultag]

Neat idea! Perhaps we can drop the compression algorithm into an exported field; cool idea, thanks!