hdf5_der

[svn-r27764] Merges r27415 from trunk:

Fix potential error with H5I_clear_type which could occur when a callback closed
a different ID in the same type.  Added a new skiplist routine,
H5SL_try_free_safe, which iterates over items, freeing some of them, and which
intercepts and defers attempts to remove from the list outside of the main
iteration.  Changed H5I_clear_type to use this function.

Tested on: ostrich and koala (h5committest)