hdf5_ep

[svn-r14238] Bug fix: It's reported by the CGNS people.  When an external link was opened twice, the library
had some trouble to close in the end.  The problem came from the field "holding_file" of 
H5O_loc_t.  It's used to indicate special links like external links.  The fix is to turn it off
whenever H5O_open is called.  Later the caller function should turn it on when it needs to.

Tested on kagiso, linew, and smirom.