sd-netlink: rename NLA_ to NETLINK_TYPE_

sd-netlink: rename NLA_ to NETLINK_TYPE_

The NLA_ names are used to name real datatypes we extract out of netlink
messages. The kernel has an internal enum with the same names
(NLA_foobar), which is *NOT* binary compatible to our types. Furthermore,
we support a different set of types than the kernel (as we try to treat
some kernel peculiarities as our own types to simplify the API).

Rename NLA_ to NETLINK_TYPE_ to make clear that this is our own set of
types.