Ensure we don't return unsuitable indicies or hardware types via RDM

[peternewman]

See the NetworkTester contents of the test suite here:
#1473 (comment)

From the standard (regarding index):

The interface identifiers returned shall range from 1 to 0xFFFFFF00.

Also add a test as a companion too.

[peternewman]

Regarding hardware types, the standard says:

The Interface Hardware Type field identifies the underlying hardware of the interface. The
values are defined in [ARP-PARAMETERS] under the 'Hardware Types' section.

So aside from working harder to fetch others (which we've already done in #1493), it's probably not worth doing anything more on that side.

[peternewman]

Other bits covered in #1504

[peternewman]

Closed in #1504 .