crit: enable python2 or python3 based crit

With this last commit of the crit with python3 series it is possible to
either use python2 or python3 with CRIU.

Now the basic build system functionality (make and make install) are
python2/python3 aware. zdtm.py and criu-coredump are still python2, but
as they are not part of 'make install' those parts have not yet been
ported from python2 to python3.

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>