Fix redirecting of IPv4 default gateway if connecting over IPv6.

Fix redirecting of IPv4 default gateway if connecting over IPv6.

Commit aa34684972eb0 fixed a long-standing bug in setting the
"route-list" flag RTSA_REMOTE_HOST for IPv4 ("we have a well-defined
remote_host == VPN server IP address") even if connecting over IPv6.

Unfortunately the logic in redirect_default_route_to_vpn() was also
wrong, and refused cooperation if that flag is not set, triggering
the message
    "NOTE: unable to redirect IPv4 default gateway -- Cannot
     obtain current remote host address"

Correct operation: if RTSA_REMOTE_HOST is not set, or remote_host
is IPV4_INVALID_ADDR (= 255.255.255.255), do not try to install a
host route for continued connectivity to the VPN server - which is
not needed when connecting over IPv6.  But the actual *routes*
(/0 or 2 x /1) can be installed just fine.

There is a second bug here, which hits if there is no IPv4 gateway
at all.  In that case, the same function triggers the message
    "NOTE: unable to redirect IPv4 default gateway -- Cannot
     read current default gateway from system"

This is caused by using "IPV4_INVALID_ADDR" as a flag for "do we
know the remote_host?" - which worked before, but after the commit
referenced above, the "remote_host" field is not well-defined unless
RTSA_REMOTE_HOST is set.  So, change the condition to check that.

Reported-By: François Kooman <fkooman@tuxed.net>
Reported-By: Thomas Schäfer <tschaefer@t-online.de>
Trac: #1332

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20201002175736.82609-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21152.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>