* PHP-8.0:
  Properly initialize PS(mod) on RINIT

* PHP-7.4:
  Properly initialize PS(mod) on RINIT

We need to do that in case a user handler has been set.  However, we
can't do that in `php_rinit_session_globals()` since that function is
called by PHP function `session_destroy()` too, but in that case we
don't want to reset PS(mod).

Closes GH-6795.

* PHP-8.0:
  Fix #80889: amendment

* PHP-7.4:
  Fix #80889: amendment

`session_set_save_handler()` may be called with callables instead of an
object; we need to cater to that as well.

We also extract a set_user_save_handler_ini() function to avoid code
duplication.

Closes GH-6796.

* PHP-8.0:
  Fix #80889: Cannot set save handler when save_handler is invalid

* PHP-7.4:
  Fix #80889: Cannot set save handler when save_handler is invalid

There is no need to require a (valid) save_handler to be set, when a
user handler is supposed to be set.  We just have to make sure, that
no user handler is already set in this case.

Closes GH-6788.

* PHP-8.0:
  Fix #80774: session_name() problem with backslash