'rebrand' OSspecific Unix -> POSIX

- change name of environment variable from $WM_OS -> $WM_OSTYPE
  as a better description

etc/controlDict

@@ -195,6 +195,7 @@ DebugSwitches
     PatchToPatchInterpolation 0;
     Phi                 0;
     PointEdgeWave       0;
+    POSIX               0;
     Prandtl             0;
     PrimitivePatch      0;
     Pstream             0;
@@ -249,7 +250,6 @@ DebugSwitches
     ThermoParcel<basicThermoParcel>     0;
     UMIST               0;
     UMISTV              0;
-    Unix                0;
     UpwindFitData<cubicUpwindFitPolynomial> 0;
     UpwindFitData<quadraticLinearUpwindFitPolynomial> 0;
     UpwindFitData<quadraticUpwindFitPolynomial>     0;