logging/ascii: Fix .shadow paths when using LogAscii::logdir

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.

testing/btest/scripts/base/frameworks/logging/rotate-leftover-log-rotation-shadow-files.zeek

@@ -0,0 +1,49 @@
+# @TEST-DOC: Enable leftover log rotation, put shadow files for conn and dns in the cwd and ensure rotation happens during startup.
+# @TEST-EXEC: echo ".log" >> .shadow.conn.log
+# @TEST-EXEC: echo "my_rotation_postprocessor" >> .shadow.conn.log
+# @TEST-EXEC: echo "leftover conn log" > conn.log
+
+# @TEST-EXEC: echo ".log" >> .shadow.dns.log
+# @TEST-EXEC: echo "my_rotation_postprocessor" >> .shadow.dns.log
+# @TEST-EXEC: echo "leftover dns log" > dns.log
+
+# @TEST-EXEC: zeek -b %INPUT > out
+
+# Ensure leftover files were removed.
+# @TEST-EXEC: ! test -f .shadow.conn.log
+# @TEST-EXEC: ! test -f conn.log
+# @TEST-EXEC: ! test -f .shadow.dns.log
+# @TEST-EXEC: ! test -f dns.log
+
+# Ensure the rotated conn log ends-up in the current working directory.
+# @TEST-EXEC: ls ./conn-*.log
+# @TEST-EXEC: cat ./conn-*.log ./dns-*.log > logs.cat
+
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-sort btest-diff out
+# @TEST-EXEC: btest-diff logs.cat
+
+module Test;
+
+export {
+	# Create a new ID for our log stream
+	redef enum Log::ID += { LOG };
+
+	# Define a record with all the columns the log file can have.
+	# (I'm using a subset of fields from ssh-ext for demonstration.)
+	type Log: record {
+		t: time;
+		id: conn_id; # Will be rolled out into individual columns.
+	} &log;
+}
+
+module GLOBAL;
+
+function my_rotation_postprocessor(info: Log::RotationInfo) : bool
+	{
+	print fmt("running my rotation postprocessor for path '%s'", info$path);
+	return T;
+	}
+
+redef LogAscii::enable_leftover_log_rotation = T;
+redef Log::default_rotation_interval = 1hr;
+redef Log::default_rotation_postprocessor_cmd = "echo";