nm_utils_ptrarray_find_binary_search() had two additional output
arguments: the first and last index -- in case the sorted list contains
duplicates.

That's nice, and was used in the past. But now, those output arguments
are no longer used.

So drop them from nm_utils_ptrarray_find_binary_search().

Actually, we could now also drop the previous variant
nm_utils_ptrarray_find_binary_search_range(), as it's only used by unit
tests. However, although not rocket science, getting this right is not
entirely trivial, so lets keep the code in case we need it again.

The libnm API NMLldpNeighbor does not accept "" as an attribute name.
And it does not need to, because a reasonable NetworkManager should
never expose such names.

However, we should not trust NetworkManager to be reasonable. Check that
the attribute name is not empty.

Move nm_lldp_neighbor_get_attr_value() first. The other functions are all redundant
and should possibly be deprecated.

There is no public API to modify a NMLldpNeighbor instance. They are only
created by NMDevice and afterwards immutable. There is no point to duplicate
them, when we can just take a ref.

Note that this means we must be careful if we ever add API to mutate a
NMLldpNeighbor. But immutable types are awesome, so we probably should
never do that anyway.

A hash table gives values in undefined order. Sort them, to
get a stable order.