Backport Shorewall 5

I’ve backported support for shorewall 5 to NethServer 6.8.
ATM, we don’t use new features, except the new persistent multiwan implementation.
Details here:

I’ve done a lot of testing with real hw and real internet uplinks without problems.
But I hope to find some testers here, to check different scenarios.

P.S. You can always find me on IRC (#nethserver on freenode) if you need help.