solvers: Added ddtCorr support in MRF regions by extending the use of Uf and rhoUf

to provide the old-time absolute flux.  This avoids possible
pressure-velocity-flux decoupling (staggering) within the MRF region using
ddtCorr to better couple the velocity and flux fields.

applications/solvers/modules/VoFSolver/VoFSolver.C

@@ -121,10 +121,13 @@ Foam::solvers::VoFSolver::VoFSolver
 {
     mesh.schemes().setFluxRequired(p_rgh.name());
 
-    if (mesh.dynamic())
+    if (mesh.dynamic() || MRF.size())
     {
         Info<< "Constructing face momentum Uf" << endl;
 
+        // Ensure the U BCs are up-to-date before constructing Uf
+        U_.correctBoundaryConditions();
+
         Uf = new surfaceVectorField
         (
             IOobject