hdf5_msb

[svn-r13497] Description:
        Move ref. count of # of links to an object out of the object header's
prefix and make it a header message instead (since it's a "rare" occurence),
eliminating some more space for each object in the file.

        Inserting this "ref. count" message exposed a flaw in the library's
mechanism for locating a message to promote to another chunk and replace
with a continuation message, which required some additional work to fix.
It's still not completely robust, but it's working for more cases now and
detects failures robustly.

        Reduced the minimum size of an object header chunk to just enough to
contain a header message prefix and continuation message.

Tested on:
	FreeBSD/32 6.2 (duty)