Add in current logfile symlink in FileBuffer

A feature like this was discussed on the previous issue https://github.com/fluent/fluentd/issues/83. The current symlink feature is something different from what I want.

**out_file** plugin uses **buf_file** plugin as buffer (in default), so new records are written to buffer at first. After a while the buffer is flushed out as logfile  by **out_file**
A matter is "what is a current logfile?"

The current symlink points a file created by **out_file**. But I think the symlink should point to a file created by **buf_file**. The feature to symlink a current buffer file can be used for not only **out_file** but also other BufferedOutput plugins.