* Many changes to make ikiwiki very resistant to write failures

* Many changes to make ikiwiki very resistant to write failures
  including out of disk space situations. ikiwiki should never leave
  truncated files, and if the error occurs during a web-based file edit,
  the user will be given an opportunity to retry.
  Inspired by the many ways Moin Moin destroys itself when out of disk. :-)
* Fix syslogging of errors.