none_t is no loner constructible from literal 0

This caused problems because:
optional<T> o = 0;
always worked. But often it would create an uninitialized optional.

test/Jamfile.v2

@@ -41,6 +41,7 @@ import testing ;
     [ compile-fail optional_test_ref_fail_init_from_Urefref.cpp ]
     [ compile-fail optional_test_ref_fail_assign_from_Trefref.cpp ]
     [ compile-fail optional_test_ref_fail_assign_from_Urefref.cpp ]
+    [ compile-fail optional_test_fail_convert_from_null.cpp ]
     [ compile-fail optional_test_fail_explicit_convert_in_value_or.cpp ]
     [ compile-fail optional_test_fail_explicit_convert_in_value_or_call.cpp ]
   ;