Implement IPv6 interface config with non-/64 prefix lengths.

Implement IPv6 interface config with non-/64 prefix lengths.

Add "ifconfig_ipv6_netbits_parm" parameter to init_tun(), use that to
initialize tt->netbits_ipv6 (previously: always /64).  Actual interface
setup code already used tt->netbits_ipv6, so no changes needed there.

Remove restrictions on "/netbits" value for --server-ipv6 config option
(can now be /64.../112, previously had to be exactly /64).  Supporting
even smaller networks could cause problems with ipv6-pool handling and
are only allowed for explicit "ifconfig-ipv6", not for "server-ipv6".

Add /netbits to pushed "ifconfig-ipv6" values on server side (client
side always accepted this, but ignored it so far, so this does not
break compatibility).

Tested on Linux/ifconfig, Linux/iproute2 and FreeBSD 7.4

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: David Sommerseth <davids@redhat.com>
Signed-off-by: David Sommerseth <davids@redhat.com>