This patch adds in support for automatically detecting and decompressing

This patch adds in support for automatically detecting and decompressing
both formats gentoo currently ships portage snapshots in. To make the
change as minimal as possible I am using recent gnu/bsd tar ability to
automatically pick the correct format and decompress. This should work
on all gentoo supported platforms including linux, freebsd, and
prefix-macosx.

Signed-off-by: Rick Farina <sidhayn@gmail.com>
Signed-off-by: Jorge Manuel B. S. Vicetto (jmbsvicetto) <jmbsvicetto@gentoo.org>