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).

bin/foamJob

@@ -23,13 +23,6 @@
 # If dispatching via foamExec
 foamExec="$WM_PROJECT_DIR/bin/tools/foamExec"
 
-# Darwin workaround - SIP clearing DYLD_LIBRARY_PATH variable
-if [ -n "$FOAM_LD_LIBRARY_PATH" ] && [ -z "$DYLD_LIBRARY_PATH" ]
-then
-    export DYLD_LIBRARY_PATH="$FOAM_LD_LIBRARY_PATH"
-fi
-
-
 #------------------------------------------------------------------------------
 
 usage() {