net-firewall/shorewall: shorewall-init added

Gentoo's shorewall-init only supports securing the firewall at boot
runlevel prior to bringing up the network.
We don't support changing the firewall state as the result of interfaces
being brought up or taken down.

For more information see the provided README.Gentoo.txt file in the
files directory.
net-firewall/shorewall: Better EPREFIX support

Previously our EPREFIX support wasn't complete. For example, shorewall
uses its own shorewallrc file, which had no EPREFIX support.

We now use the 'eprefixify' function from Gentoo's prefix.eclass to adjust
shorewallrc while installation.

I also updated shorewallrc itself.

Finally, because I had to modify shorewallrc in $S, I changed the way we
install shorewall. We now make more usage of upstream's install.sh.