This makes newsyslog use zstandard to compress log files.
Given Z is already taken for gzip and zstandard compression level stands in
between gzip and xz (which has the X flag) chosing Y sounds ok :)
Add the much-needed and long-desired ability to force the log rotation
to take place on a specific day or time, and document same. This works
by extending the syntax of the `when' field to allow a restricted
ISO 8601 time specification, and performing the log rotation if newsyslog
is run during the same hour as is specified.
