Various other functions/structures are now only used in criu.c, so let's
hide stuff there so as not to pollute headers.

This commit also bumps the required CRIU versions to 2.0. While we don't
*require* any features that aren't in 1.8 patchlevel 21 or above, 2.0 is a
vast improvement, and so we should use that instead.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>