hdf5_ep

[svn-r5191] Purpose:
    Bug fix

Description:
    When several level deep nested compound & VL datatypes are used, the data
    in the nested compound datatypes is incorrectly sharing the same "background
    buffer", causing data corruption when the data is written to the file.

Solution:
    Allocate a separate background buffer for each level of the nested types
    to convert.  (Also allocate temporary background buffers for array
    datatypes, where this sort of problem could occur also)

    Added more regression tests to check for these errors.

Platforms tested:
    FreeBSD 4.5 (sleipnir) & Solaris 2.6 (baldric)