Make logfile's mode configurable.

The file logger currently resets the mode of the logfile to 0640.

When the webserver is running as a different user than the cron job
(but both are in the same group) the files mode has to be 0660. The
current implementation breaks logging for the user that is not the
owner of the logfile.

This patch introduces a new config option 'logfilemode' that expects
an octal value (defaults to 0640). Unless the value is lower or equal
than 0 the logfiles mode will be resetted to this value.

Signed-off-by: Roland Tapken <roland@bitarbeiter.net>