[FS#978] netifd: support smaller ipv6 segments than /64

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Nathaniel Wesley Filardo (nwf) 

Attached to Project - LEDE Project
Summary - netifd: support smaller ipv6 segments than /64
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - netifd makes the assumption that ipv6 segments will never be any smaller than a /64; it quite often computes things like (1 length)) which obviously won't work out very well when prefix->length > 64.  (Incidentally, it looks like it's never been tested with prefixes larger than a /32, too, since things like "int32_t current = 0, asize = (1 length)) - 1;" will also not work out very well when assign->length