ENH: lazier handling of dynamic libraries

- previously always called dlclose on opened libraries when destroying
  the dlLibraryTable. However, by force closing the libraries the
  situation can arise that the library is missing its own code that it
  needs on unload (#1524). This is also sometimes evident when closing
  VTK libraries for runTimePostProcessing (#354, #1585).

- The new default is to not forcibly dlclose any libraries, unless
  the dlcloseOnTerminate OptimisationSwitch specifies otherwise.

  - The dlLibraryTable::close() method can be used to explicitly close
    all libraries and clear the list.

  - The dlLibraryTable::clear() method now only clears the entries,
    without a dlclose.