Default BINPKG_COMPRESSION to zstd (bug 715108)

This includes a _compat_upgrade.binpkg_compression script that the
ebuild can call in pkg_preinst in order to maintain a backward-compatible
bzip2 default when appropriate, ensuring that binary package consumers
are not caught off guard.

Bug: https://bugs.gentoo.org/715108
Reviewed-by: Brian Dolbec <dolsen@gentoo.org>
Signed-off-by: Zac Medico <zmedico@gentoo.org>
5 files changed