Clean some header related things.

Revert most of "Hack together a fix for the public headers"

- The `libmain` change is kept, and one more libmain change is made.
  (Need to update Meson and Nix per the package alike).

- The S3 situation is fixed in a different way: the variable is public
  now, used in the header, and fixed accordingly.

- Fix TODO for `HAVE_EMBEDDED_SANDBOX_SHELL`

This reverts commit 2b51250534.

src/libstore-tests/s3-binary-cache-store.cc

@@ -1,10 +1,9 @@
-#include "store-tests-config.hh"
-#if ENABLE_S3
+#include "nix/store/s3-binary-cache-store.hh"
+
+#if NIX_WITH_S3_SUPPORT
 
 #  include <gtest/gtest.h>
 
-#  include "nix/store/s3-binary-cache-store.hh"
-
 namespace nix {
 
 TEST(S3BinaryCacheStore, constructConfig)