hosts.lisp: Replacing IPv6 host routes with /112 networks.

hosts.lisp: 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.