sd-netlink: rename NLType and friends

sd-netlink: rename NLType and friends

In sd-netlink, we use 'type' for naming many variables with different type,
and worst of all, function like `type_get_type()` exists. That makes the
code hard to understood.

Let's rename
- NLType -> NLAPolicy
- NLTypeSystem -> NLAPolicySet
- NLTypeSystemUnion -> NLAPolicySetUnion
- type_get_type() -> policy_get_type()
- type_get_type_system() -> policy_get_policy_set(), and so on.

This also changes the type of attribute taken by e.g.
`sd_netlink_message_append_string()` from `unsigned short` -> `uint16_t`
to make the variable size clearer.