Description:
    - Added another overload for char* argument:
        ssize_t getComment(const char* name, const size_t buf_size, char* comment)
    - Changed default value to 0 for the other two getComment methods
    - Added HDmemset to after every char string allocation to clear the buffer
Platforms tested:
    Linux/ppc64 (ostrich)
    Linux/32 2.6 (jam)
    Linux/64 2.6 (platypus)/PGI compilers



git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@24865 dab4d1a6-ed17-0410-a064-d1ae371a2980