inet6: Don't install a default route if no global address

inet6: Don't install a default route if no global address

This works around an issue where some routers advertise themselves
as a default router, but offer no address to actually use.
Thus, it is actually useless as a router.
If an address is added by any other means (DHCPv6, static, etc)
then the default route is installed once more.

This is also dynamic - dhcpcd reacts to addresses being added,
removed, duplicated, detached, etc.