Remove the "preview" status from the loaded object feature

Remove the "preview" status from the loaded object feature

Add an ABI version both to the header file and passed to the setup
function.  Unfortunately this itself is an ABI break and I couldn't
find a good way to avoid it.

* NEWS: Announce the ABI is not a preview and the incompatibility.
* doc/make.texi: Remove the preview warnings for object loading.
Document the new ABI version argument.
* src/gnumake.h (GMK_ABI_VERSION): Set the ABI version to 1.
Add comments documenting the format of the setup function.
* src/load.c (setup_func_t): Rename from load_func_t.
(load_file): Pass the ABI version to the setup function.
* tests/scripts/features/load: Rework the setup function.
* tests/scripts/features/loadapi: Ditto.