widl: Consolidate writing of COM and dispatch interfaces into one function to remove duplicated code.

Split up the writing into start and end to eventually support the style 
MIDL uses where it writes declared types, etc. between the start and end 
of the interface.

Make internal header functions take the file pointer to print to.

Don't write interface IDs for non-object interfaces and always write 
handle declarations even if the interface has no methods, like MIDL does.

Go back to not mapping zero-size windows.

Based on a patch by Roderick Colenbrander.

Instead just revert the position slightly, this is a lot easier to maintain.

After a flush call there should be no more samples in the buffer.

This reverts commit 1ba8ece7.

Code for holding sample doesn't make sense and shouldn't be there
because it should really be handled by keeping track of the last
successful position of a frame sent and aborting the whole processing
as soon as rejection occurs.