vchoi_fork

[svn-r12596] Refactored how external files are opened and closed.

Object header locations (H5O_loc_t's) can now "hold open" a file and
decrement its open object count when they close.  This means that
locations (H5O_loc_t's and H5G_loc_t's) should always be freed.

Added more thorough tests to ensure that external files are closed.