* log: Only chmod if permission bits differ
Follow-up to #6314 and https://caddy.community/t/caddy-2-9-0-breaking-change/27576/11
* Fix test
* Refactor FileWriter
* Ooooh octal... right...
101d3e7 introduced a configuration option to set the log file mode.
This option was not taken into account if the file already exists,
making users having to delete their logs to have new logs created
with the right mode.