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_i

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.

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