Also modify FormatRotationPath to keep rotated logs within
Log::default_logdir unless the rotation function explicitly
set dir, e.g. by when the user redef'ed default_rotation_interval.
With the introduction of LogAscii::logdir, log filenames can now include
parent directories rather than being plain basenames. Enabling log rotation,
leftover log rotation and setting LogAscii::logdir broke due to not
handling this situation.
This change ensures that .shadow files are placed within the directory where
the respective .log file is created. Previously, the .shadow. (or .tmp.shadow.)
prefix was simply prepended, yielding non-sensical paths such as
.tmp.shadow.foo/bar/packet_filter.log for a logdir of foo/bar.
Additionally, respect LogAscii::logdir when searching for leftover log files
rather than defaulting to the current working directory.
The following quirk exist around LogAscii::logdir, but will be addressed
in a follow-up.
* By default, logs are currently rotated into the working directory of the
process, rather than staying confined within LogAscii::logdir. One of
the added tests shows this behavior.
