rename Foam::dotFoam() -> Foam::findEtcFile() with an optional 'mandatory' argument

- if mandatory is true, findEtcFile() will abort with a message (via cerr).
  This allows a non-existent file to be caught at the lowest level and avoid
  error handling in IFstream, which might not be initialized at that stage.

src/OpenFOAM/primitives/strings/string/string.C

@@ -202,7 +202,7 @@ Foam::string& Foam::string::expand()
             // otherwise add extra test
             if (user == "OpenFOAM")
             {
-                *this = dotFoam(file);
+                *this = findEtcFile(file);
             }
             else
             {