Examine IA_NA prefix length values in our code for consistency and usability

The following support ticket: https://support.isc.org/Ticket/Display.html?id=13645, raises an issue with our inconsistent handling of prefix length for IA_NA leases and reservations within the code base.

While the immediate issue raised by the ticket seems to be mitigated by changes made for Global Host Reservations, we should still spend time analyzing our code for both consistency and usability. Primarily we to decide if, we allow values of both 0 and 128, or only 128.

This may require polling the user community.