Use the newly added GetSystemDefaultInterfaceOrFallback() to return
actual information for the default interface, not just dummy
interface details.
This also fixes GetInterfaceOpt(0x4003) and gethostid() returning
inconsistent information. Prior to this change, GetInterfaceOpt(0x4003)
would return 10.0.1.30 and gethostid would give a totally unrelated IP.