hdf5_msb

[svn-r20691] Purpose: Fix problem exposed by external link cache

Description:
When closing an object that had two open id's, the library would previously fail
to account for the status of the "holding_file" field, potentially causing the
file's "nopen_objs" field to become inconsistent.  This caused problems when
opening the same object twice through an external link using th external file
cache.  Modified the library to properly account for the status of the
"holding_file" field when closing an object id, even when the underlying
"shared" object isn't closed.

Tested: jam, koala, heiwa (h5committest)