Follow disable_ipv6 for Pulse and Fortinet

Follow disable_ipv6 for Pulse and Fortinet

As with other protocols (AnyConnect, F5, GP), the behavior of 'disable_ipv6'
for these protocols is relatively "shallow": if set, it will cause
OpenConnect to ignore any IPv6 address or netmask sent by the server, but
will *not* ignore IPv6 split-{in,ex}cludes or IPv6 addresses of DNS servers.

More thorough IPv6-ignoring could be handled by the vpnc-script, or cleaned
up as part of a future change to simplify IP configuration and routing
across protocols.

(The lack of support for --disable-ipv6 in Pulse was noted in
https://gitlab.com/openconnect/openconnect/-/issues/254.)

Signed-off-by: Daniel Lenski <dlenski@gmail.com>