Fix incompatibility with openfortivpn >= 1.11.0

Before openfortipvn 1.11.0 the default configuration was equivalent to:

--set-dns=1 --pppd-use-peerdns=1

Starting with openfortivpn 1.11.0 the default configuration is equivalent to:

--set-dns=1 --pppd-use-peerdns=0

NetworkManager-fortisslvpn expects the configuration to be equivalent to:

--set-dns=0 --pppd-use-peerdns=1

This expectation breaks with openfortivpn 1.11.0 because NetworkManager-fortisslvpn only passes this command line option:

--no-dns / --set-dns=0

Starting with openfortivpn 1.11.0 it needs to be:

--no-dns / --set-dns=0 --pppd-use-peerdns=1

This patch adds --pppd-use-peerdns=1 as is already the case in Fedora packages:

https://src.fedoraproject.org/rpms/NetworkManager-fortisslvpn/c/6378487

This is also explained here:

https://github.com/adrienverge/openfortivpn/issues/503