Create network directory if necessary

src/freedombone-utils-firewall

     iptables-save > /etc/firewall.conf
     ip6tables-save > /etc/firewall6.conf
     if [ ! -d /etc/network/if-up.d ]; then
-        mkdir /etc/network/if-up.d
+        mkdir -p /etc/network/if-up.d
     fi
     printf '#!/bin/sh\n' > /etc/network/if-up.d/iptables
     printf 'iptables-restore < /etc/firewall.conf\n' >> /etc/network/if-up.d/iptables