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param: [sip,cnx_ip_to_0000_if_sendonly_enabled]  with default value 0

- int values are accepted where a long is expected.
- int or long values are accepted where a float is expected.

- Add the LinphoneBuffer object so that file transfer callbacks do not pass out parameters
- Use the LinphoneBuffer object in the file transfer callbacks
- Implement the file_transfer_send callback in the Python file transfer test

Reinitialize log serialization only when all the linphone cores have been destroyed.