hdf5_ep

[svn-r18619] Description:
	Bring r18618 from trunk to 1.8 branch:

	Add new internal object header routines to query the header chunk that
a message is in, and to lock/unlock a message into a chunk (so it doesn't get
moved into another chunk during allocation/free events).

       Also, move dataspace message to beginning of object header messages
added to a dataset's object header, so it can be locked into chunk #0 (when
performing SWMR operations).

Tested on:
       FreeBSD/32 6.3 (duty) in debug mode
       FreeBSD/64 6.3 (liberty) w/C++ & FORTRAN, in debug mode
	(h5committested on trunk)