net-firewall/shorewall-4.5.18-r1 release prepared

shorewall-4.5.18 was stabilized last week but contains a bug,
which will prevent shorewall from working, when the user did not call
the init.d script before, see bug #459316.

This revision will backport the fix from the 4.5.19 release.

The initscript itself was tweaked, too, since the 4.5.18 release
from Constanze.