Cleanup config headers

There are two big changes:

1. Public and private config is now separated. Configuration variables
   that are only used internally do not go in a header which is
   installed.

   (Additionally, libutil has a unix-specific private config header,
   which should only be used in unix-specific code. This keeps things a
   bit more organized, in a purely private implementation-internal way.)

2. Secondly, there is no more `-include`. There are very few config
   items that need to be publically exposed, so now it is feasible to
   just make the headers that need them just including the (public)
   configuration header.

And there are also a few more small cleanups on top of those:

- The configuration files have better names.

- The few CPP variables that remain exposed in the public headers are
  now also renamed to always start with `NIX_`. This ensures they should
  not conflict with variables defined elsewhere.

- We now always use `#if` and not `#ifdef`/`#ifndef` for our
  configuration variables, which helps avoid bugs by requiring that
  variables must be defined in all cases.

tests/functional/plugins/meson.build

@@ -1,13 +1,6 @@
 libplugintest = shared_module(
   'plugintest',
   'plugintest.cc',
-  cpp_args : [
-    # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
-    # It would be nice for our headers to be idempotent instead.
-    '-include', 'nix/config-util.hh',
-    '-include', 'nix/config-store.hh',
-    '-include', 'nix/config-expr.hh',
-  ],
   dependencies : [
     dependency('nix-expr'),
   ],