[ipv4] Abstract out protocol-specific portions of "route" command

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[ipv6] Replace IPv6 stack

Replace the existing partially-implemented IPv6 stack with a fresh
implementation.

This implementation is not yet complete.  The IPv6 transmit and
receive datapaths are functional (including fragment reassembly and
parsing of arbitrary extension headers).  NDP neighbour solicitations
and advertisements are supported.  ICMPv6 echo is supported.

At present, only link-local addresses may be used, and there is no way
to specify an IPv6 address as part of a URI (either directly or via
a DNS lookup).

Signed-off-by: Michael Brown <mcb30@ipxe.org>