hdf5_ep

[svn-r16473] Purpose: fix problems related to 'self-referential' attributes

Description:
When an attribute was created with a datatype or dataspace that was shared in
the same object header that the attribute was in, the attribute could not be
deleted.  Changes made to ensure that the attribute can be deleted both when the
attribute is in the object header and when it is shared in the heap.  Object
header message decode routines now take an "open_oh" parameter to enable them to
avoid opening the same object header twice.

Tested: jam, smirom (h5committest)