Log file: fix the duplication of lines in log file when...

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 *file_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 log handler of the previous log file in service->file_log_handler and set the file log handler of the new log handler.
If filename is NULL, close the current log file and return.

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.