Enhance IPv6 block on Eglide

Summary:
To have a more logic block, we fill the gaps in the ::1-::19 series with hexadecimal ones.

Furthermore, currently, Debian tends to select last added address,
currently inet6 2001:470:1f13:896::16 (at the top of ifconfig).

Aliases are now RFC 3484 compliant with preferred_lft set at 0, so
it will normally prefer the canonical eglide.org ::code:15:11fe one.

Test Plan: ifconfig sit1

Reviewers: Sandlayth, dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D889