bpo-39796: Fix _warnings module initialization (GH-18739)

* Add _PyWarnings_InitState() which only initializes the _warnings
  module state (tstate->interp->warnings) without creating a module
  object
* Py_InitializeFromConfig() now calls _PyWarnings_InitState() instead
  of _PyWarnings_Init()
* Rename also private functions of _warnings.c to avoid confusion
  between the public C API and the private C API.