ENH: avoid faPatch/fvPatch patchInternalField ambiguity

- with alternative faceCell addressing, use the three-parameter
  version only. This avoids potential future ambiguity with the
  two-parameter version (eg, with a label type)

ENH: add faPatchField patchInternalField() for symmetry with fvPatchField

ENH: direct reference to mesh thisDb instead of inferring

ENH: pointMesh::boundaryMesh() method (eg, similar to fvMesh)

src/finiteArea/faMesh/faPatches/constraint/processor/processorFaPatch.C

@@ -492,7 +492,9 @@ Foam::tmp<Foam::labelField> Foam::processorFaPatch::interfaceInternalField
     const labelUList& edgeFaces
 ) const
 {
-    return patchInternalField(internalData, edgeFaces);
+    auto tpfld = tmp<labelField>::New();
+    patchInternalField(internalData, edgeFaces, tpfld.ref());
+    return tpfld;
 }