logging: Introduce Log::default_logdir deprecate LogAscii::logdir and per writer logdir

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.

testing/btest/scripts/base/frameworks/logging/rotate-default-logdir.zeek

@@ -0,0 +1,34 @@
+# @TEST-DOC: Set Log::default_logdir to ./logs. Ensure logs stay within ./logs.
+# @TEST-EXEC: mkdir logs
+# @TEST-EXEC: zeek -b -r ${TRACES}/rotation.trace %INPUT >zeek.out 2>&1
+# @TEST-EXEC: grep "test" zeek.out | sort >out
+# @TEST-EXEC: for i in `ls ./logs/test.*.log | sort`; do printf '> %s\n' $i; cat $i; done >>out
+# @TEST-EXEC: btest-diff out
+
+module Test;
+
+export {
+	# Create a new ID for our log stream
+	redef enum Log::ID += { LOG };
+
+	# Define a record with all the columns the log file can have.
+	# (I'm using a subset of fields from ssh-ext for demonstration.)
+	type Log: record {
+		t: time;
+		id: conn_id; # Will be rolled out into individual columns.
+	} &log;
+}
+
+redef Log::default_logdir = "./logs";
+redef Log::default_rotation_interval = 1hr;
+redef Log::default_rotation_postprocessor_cmd = "echo";
+
+event zeek_init()
+	{
+	Log::create_stream(Test::LOG, [$columns=Log]);
+	}
+
+event new_connection(c: connection)
+	{
+	Log::write(Test::LOG, [$t=network_time(), $id=c$id]);
+	}