Add ipfilter support to blacklistd-helper

[• lidl]

Summary

This is a debugged version of Cy's patch for ipfilter support
that I have tested on a virtual machine.

It also merges in a couple of small changes that Christos made to the NetBSD version
of the file. A few long lines were wrapped. I also squashed some trailing whitespace.

Test Plan

Tested on a virtual machine. It works for adding and removing rules.

[cy]

Looks good.