hdf5_ep

[svn-r7538] Purpose:
    Bug fixes and code cleanup

Description:
    Lots of changes here:
        - Fixed bug #691 - when shared datatypes are used in attributes they
            are incorrectly copied into the attribute instead of referring
            the the named datatype in the file.  This required bumping the
            version of the attribute message.  The new version of the attribute
            message is only written out when a shared datatype is used in
            the attribute.  [Also, this format change made the size of the
            attribute smaller.]
        - Added information to attribute debugging routine so that shared
            datatypes are displayed correctly with the h5debug tool.
        - Refactored the H5O* routines to extract code that was common to
            several routines into subroutines to call.
        - Added 'link' method for H5O message sub-classes, which increments
            the link count on shared objects when a message is created which
            shares them.
        - Corrected [unreported] bug where the link count was not being
            decremented on the shared object when a object header message
            with a reference to that object was deleted from the file.
        - Reduced size of shared message from 49 bytes (which was incorrect
            anyway and should have been 48 bytes) to 10 bytes, which required
            bumping the version of "shared" messages.
        - Refactored some of the shared datatype routines to allow for easier
            queries of "committedness" internally to the library and also
            added routine to easily increment/decrement the reference count of
            a shared datatype.

Platforms tested:
    FreeBSD 4.9 (sleipnir)
    h5committest