sd-netlink: turn 'max' into 'count' to support empty type-systems

Right now we store the maximum type-ID of a type-system. This prevents us
from creating empty type-systems. Store the "count" instead, which should
be treated as max+1.

Note that type_system_union_protocol_get_type_system() currently has a
nasty hack to treat empty type-systems as invalid. This might need some
modification later on as well.