Test if $_SESSION['country_code'] is set in _country_code_init_code()

In _country_code_init_code(), we should be considering whether the country_code session variable is already set as part of the logic of determining how to set it.

This will allow us to distinguish between fresh visits and repeat ones. This is related to the existing test for whether a request originates externally. If this is a new request, we should behave as if it is external.