Remove unused variables.
Reorder operations to be the same as in fuse.h.
Remove unused and essentially empty gridfs_mknod function.
Implement directories.

Directories are created in the same collection as files, except:
 - their mode contains S_IFDIR instead of S_IFREG
 - their length and md5 are set to 0
 - they obviously have no chunks in the chunks collection
I don't know what effect this will have on other GridFS software.
This hasn't been tested much.
Should 'mode' be a descriptive data structure (hash) instead of a straight bitmask?