implement adding/deleting routes on AIX, for IPv4 and IPv6

implement adding/deleting routes on AIX, for IPv4 and IPv6

AIX only has TAP interfaces, so always use gateway address as next hop,
not interface name.

AIX route works much more reliable if passed netbits than netmask - do so
(introducing a new helper function netmask_to_netbits2())
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1402409073-54067216-4-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8785

Signed-off-by: Gert Doering <gert@greenie.muc.de>