Working on the logging API exposed to scripts.

- Moving all functions into the Log::* namespace, using the recent
  bifcl updates. Moved logging-specific stuff to logging.bif.

- Log::create_stream() now takes a record Log::Stream as its second
  argument, which specifies columns and (optionally) the event.

- All the internal BiFs are now called "Log::__<something>", with
  script-level wrappers "Log::<something>". That first allows to add
  additional code at the script-level, and second makes things better
  comprehendible as now all relevant functionality is collected (and
  later documetned) in policy/logging.bro.

- New function Log::flush(id), which does the obvious assuming the
  writer supports it.

- add_default_filter() is now called implicitly with every
  create_stream(). Seems that we usually want that functionality, and
  when not, remove_default_filter() gets rid of it.

- The namespace of a stream's ID is now used as the default "path"
  (e.g., if the namespace is SSH, the default log file is "ssh.log").

- Updated policy/test-logging.bro as well as the btest tests according
  to these changes.

src/LogWriterAscii.h

@@ -19,6 +19,7 @@ protected:
     virtual bool DoWrite(int num_fields, LogField** fields, LogVal** vals);
 	virtual bool DoSetBuf(bool enabled);
 	virtual bool DoRotate(string rotated_path);
+	virtual bool DoFlush();
     virtual void DoFinish();
 
 private: