Add SELECTIONBAR to mask, and use settings rather then lParam. 2 subtle
bugs that prevent selectionbar from working.

Fix up a couple of conflicts with the Status field in the
RPC_EXTENDED_ERROR_INFO and the X Status define.

Support handing out association group IDs.

There are other cases where calls to Free functions can be omitted (such 
as types always unmarshalled using buffer memory with no embedded 
pointers), but these are easier dealt with inside the relevent case in 
write_remoting_arg.

This partially reverts commit 6e9fea22.
16-bit and 32-bit clipboard handles aren't interchangeable.