bazil.org/fuse: Add HandleReadDirer to read partial sub-dirents

This patch introduces an Interface HandleReadDirer, which implements
ReadDir to read partial sub-dirents accroding to request offset and
size.

From now on, readdir could be implemented in two ways:
 * ReaddirAll reads all sub-dirents one time
 * Readdir reads partial sub-dirents and the caller needs to call
   it multple times if more sub-dirents are requested
 * If both are implemented, Readdir is used

Signed-off-by: Sheng Yong <[email protected]>