the catalog database keeps track of three mappings:
    CNID     -> dev/ino and did/name
    dev/ino  -> CNID
    did/name -> CNID

dev/ino is used to keep track of magically moved files. did/name is
for quick lookups of CNIDs. 

NOTE: the database will append a nul byte to the end of name. in
addition, name should be given as it appears on disk. this allows the
creation of cnid updating/cleaning programs that don't have to deal
with knowing what the particular codepage is.

here's the ritual:
	1) open a volume. call cnid_open.
	2) every time you need a CNID, call cnid_add(). it will
	   automatically look for an existing cnid and add a new one
	   if one isn't already there. you can pass a hint if you
	   want. the only use this has right now is to enable
	   consistency between AFP and HFS. in the future, it would
	   allow people to write conversion utilities that
	   pre-instantiate a database without needing to re-assign
	   CNIDs.
	3) if you want to just look for a CNID without automatically
	   adding one in, you have two choices:
	     a) cnid_resolve takes a CNID, returns name, and
	        over-writes the CNID given with the parent DID. this
	        is good for FPResolveID.
             b) cnid_lookup returns a CNID corresponding to the
	        dev/ino,did/name keys. it will auto-update the catalog
		database if there's a discrepancy. 
		NOTE: cnid_add calls this before adding a new CNID. 
	4) when you delete a file or directory, you need to call
	   cnid_delete with the CNID for that file/directory.
	5) if you're just moving a CNID, call cnid_move with the CNID
	   and the new dev/ino,did/name pairs.
	6) call cnid_close when closing the volume.
