local.m4: Replacing IPv6 host routes with /112 networks.

local.m4: Replacing IPv6 host routes with /112 networks.

Linux has a bug: it doesn't make route cache entries for remote hosts
if there's already a host route, and it only attaches path-MTU
information to cache entries.  The result is that it doesn't handle
ICMPv6 `packet too big' messages properly for destinations with host
routes.

I'm bodging this by replacing all of the host routes with tiny /112
networks.  It's awful, but it seems to work.  The convention is that
the `host part' of the net is always zero.