Directory operations

[sandrain]

In UnifyCR, the directory operations are only handled locally, which may cause a inconsistent status to applications. In specific, the following directory operations are expected to be implemented:

• opendir(3)
• fdopendir(3)
• closedir(3)
• readdir(3)
• rewinddir(3)
• dirfd(3)
• telldir(3)
• scandir(3)
• seekdir(3)

One thing to discuss is that there might be a case where such globally-synchronized directory operations are not necessary but prefers better performance by not touching the global metadata db for directory operations. Although directory operations would not be a huge I/O bottleneck in most cases, it would be possible to make this as configurable by the consistency model that user specifies (i.e., laminated, posix, ....).