int32 with boolean value in protobuf has the same size with bool,
many sysctls are boolean but we don't lose anything by storing them
in int32, so add only int32 and string fields

will need string field for stable_secret ipv6 sysctl

also such fromat allows us to easily handle non-present int sysctls
we can check if we have it using has_*arg

v3: rebase images/Makefile to criu-dev branch
v4: use enum for type
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Reviewed-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>