implement disabling of log rotation

* Use “MiB” instead of “MByte” to clarify that the base 1024 is used.
* Implement disabling of the logrotation by setting max_logfile_size=0.
  Please check whether this is correct:
  write_log() seems to return 0 on errors and filelen otherwise.
  do_log() however, interprets a return value of 0 as OK and everything else as ERROR.
* Added the new functionallity to the documentations.