hdf5_ep

[svn-r21646] Port r21603 to 1.8 branch

Purpose: Add generic skip list implementation

Description:
Added new H5SL_TYPE_GENERIC skip list type, which uses void *'s as keys and a
client-supplied callback for key comparison.  This was added to support the
upcoming "merge named datatype" feature for H5Ocopy, but may be used in other
places as well.  Also added testing.

Also fixed a potential bug with the H5SL_TYPE_OBJ implementation, and added
testing for that.

Tested: jam, koala, heiwa (h5committest), durandal