comdlg32: Add a placeholder for the unicode dialog struct. Update helper functions to work in the unicode case.

It does more harm than good, since we can't get any information about
what new features are being used by the caller, if any, by bailing at
this early point.

NdrRangeMarshall, NdrRangeBufferSize, NdrRangeFree and
NdrByteCountPointerMemorySize should be declared static in
ndr_marshall.c instead.

Moved and duplicated the code below the "if child" statement into the
if and else block.