sd-netlink: make NLTypeSystem internal

sd-netlink: make NLTypeSystem internal

Same as NLType, move NLTypeSystem into netlink-types.c and hide it from
the outside. Provide an accessor function for the 'max' field that is used
to allocate suitable array sizes.

Note that this will probably be removed later on, anyway. Once we support
bigger type-systems, it just seems impractical to allocate such big arrays
for each container entry. An RBTree would probably do just fine.