hdf5_msb

[svn-r18374] Purpose: Fix bugs involving external links

Description:
Previously, the library would reopen the source file when traversing an external
link if unable to find the target file otherwise.  This has been corrected.
Also moved the call to H5F_flush from H5F_try_close to H5F_dest, so the file is
only flushed when the last identifier for the file is closed.  This prevernts
situations where the library could attempt to flush a file with protected
metadata.

Tested: jam, amani, linew (h5committest); Fedora