hdf5_msb

[svn-r13753] Description:
	Check in "unique, but sharable" optimization to ISOHM code, which
allows object header messages that are only used in one object to remain in
the sole user's header, but migrates messages that are used in more than one
header into the ISOHM heap.

Tested on:
	Mac OS X/32 10.4.9 (amazon)
	FreeBSD/32 6.2 (duty)
	Linux/32 2.6 (chicago)
	Linux/64 2.6 (chicago2)