cconv.el: Fix regression in cconv-tests-interactive-closure-bug51695

cconv.el: Fix regression in cconv-tests-interactive-closure-bug51695

The new code to make interpreted closures safe-for-space introduced
a regression in `cconv-tests-interactive-closure-bug51695`, only seen
when using TEST_LOAD_EL.
A few other issues were found and fixed along the way.

* lisp/emacs-lisp/cconv.el (cconv-fv): Change calling convention and
focus on finding the free variables.
(cconv-make-interpreted-closure): New function.

* lisp/loadup.el: Use `compiled-function-p` rather than
`byte-code-function-p` so we also use safe-for-space interpreted
closures when we build with native compilation.
(internal-make-interpreted-closure-function):
Use `cconv-make-interpreted-closure`.

* src/eval.c (syms_of_eval): Rename `internal-filter-closure-env-function`
to `internal-make-interpreted-closure-function`.
(Ffunction): Let that new var build the actual closure.

* test/lisp/emacs-lisp/cconv-tests.el
(cconv-tests-interactive-closure-bug51695): Test specifically the
interpreted case.