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.

(cherry picked from commit 3294b22a68)

src/libstore-tests/meson.build

@@ -40,9 +40,6 @@ deps_private += gtest
 configdata = configuration_data()
 configdata.set_quoted('PACKAGE_VERSION', meson.project_version())
 
-aws_s3 = dependency('aws-cpp-sdk-s3', required : false)
-configdata.set('ENABLE_S3', aws_s3.found().to_int())
-
 config_priv_h = configure_file(
   configuration : configdata,
   output : 'store-tests-config.hh',