ENH: reimplement Foam::dlOpen() for macOS (#2801)

- The Apple SIP (System Integrity Protection) clears environment
  variables, which affects the behaviour of dynamic library loading
  (the DYLD_LIBRARY_PATH env variable).

  OpenFOAM shadows this variable as FOAM_LD_LIBRARY_PATH, which has
  been used to restore DYLD_LIBRARY_PATH (eg, in RunFunctions script).

  However, this solution is not quite complete, as it
    (a) requires sourcing of RunFunctions file,
    (b) additional errors appear depending on a user workflow.

  This changeset alleviates the problem by also iterating through
  paths stored in the shadow variable when loading dynamic libraries
  (if the DYLD_LIBRARY_PATH is empty).