(struct directory.contents, flags): New members

(nfs,found,new): Remove. Replaced by appropriate bitmask values in
`flags' field. All uses updated.
(directory_meta_table): New table.
(hash_directory): Rename to hash_directory_name
(compare_directories): Rename to compare_directory_names
(hash_directory_meta,compare_directory_meta,find_directory_meta):
New functions
(compare_dirents): Removed
(note_directory): Get 7th argument: directory contents.
All callers updated
(dumpdir_locate,makedumpdir): New functions
(scan_directory): Rewritten. Use makedumpdir to create a sorted
dumpdir array. This makes the obstack argument unnecessary. Besides,
ALL_CHILDREN flag is set only for new directories.
(procdir): Change return type to struct directory. Return
immediately if the directory was already initialized. Discover
directory renames using directory_meta_table.
(append_incremental_renames): New function.
(read_directory_file, write_directory_file): Use new snapshot file
format.