Log file: fix the duplication of lines in log file when linphone_logging_service_set_log_file() is called several times.
Add the field bctbx_log_handler_t *previous_log_handler in LinphoneLoggingService to be able to close the current log file when a new one is set. In linphone_logging_service_set_log_file(), destroy the file handler of the previous log file in service->previous_log_handler and set the file handler of the new log handler.
Add a suite to check that the logs are written in only once in the current log file, even when linphone_logging_service_set_log_file is called() several times. This suite must be run with the --verbose option.
Ticket Mantis 0013177 : https://mantis.linphone.org/view.php?id=13177