Make this thing functional...

JSON data is now loaded and used at runtime.
Stole a bunch of gdnsd's well-tested code for the
  network data structure stuff, in subdir src/nlt/
JSON file is now set per-VCL via netmapper.init()
  in vcl_init(), so that the module can be used
  for multiple things on the same server...

Known issues:
  Docs/copyright/packaging/whatever.
  Error output is less than ideal in places.
  I'm still not 100% sure varnish won't double-init
    the code, may require minor cleanup.
  The JSON reloader code is currently unconditionally
    reloading the JSON at every check interval,
    it should track stat(2) results and not reload
    when the file hasn't changed, obviously.  Kinda
    nice for torture testing some of those tricky
    bits, though.

Change-Id: I05e88a332248cb4eb1da8477f703c9d616cd1542