Allow learning iroutes with network made up of all 0s (only if netbits < 8)

Allow learning iroutes with network made up of all 0s (only if netbits < 8)

It is plausible for a user to be willing to add a route for a network
made up of all 0s via a VPN client (i.e. 0.0.0.0/1), therefore such
iroute should be supported.

As of now the option parsing code will accept such iroute, but
the learning routine will (silently) reject it after a sanity check.

Such check prevents routes with network made up of all 0s to be
learnt at all..

Change the sanity check so that it will reject iroutes to network
made up of 0s only when netbits is greater than 7.

The reason for choosing 7 is because anything within 0.0.0.0/8 is not
really routable among networks.

While at it, make the sanity check louder so that it can print the
reason why a route is being rejected.

Trac: #726
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20171206154356.30764-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16044.html
Signed-off-by: David Sommerseth <davids@openvpn.net>