Basic support for IPv6. The system-dependent part doesn't work yet,

Basic support for IPv6.  The system-dependent part doesn't work yet,
but the core routines are there and seem to be working.

   o  lib/ipv6.[ch] written
   o  Lexical analyser recognizes IPv6 addresses and when in IPv6
      mode, treats pure IPv4 addresses as router IDs.
   o  Router ID must be configured manually on IPv6 systems.
   o  Added SCOPE_ORGANIZATION for org-scoped IPv6 multicasts.
   o  Fixed few places where ipa_(hton|ntoh) was called as a function
      returning converted address.