Suggested fix for JDK-4724038 (Add unmap method to MappedByteBuffer)

[Mike Hearn]

>
> So Windows has no operation which can remap a previously mapped section of
> memory to something else? (say, just a block of non-access memory)
>
> Note that such an operation has to be atomic with respect to all other
> operations which affect the address space of the process (or at least,
> which may randomly choose an address in memory at which to create new
> mappings), but not with respect to anything else really.  The point of the
> remapping is to continue to reserve the address space, without actually
> referencing what was there before (thus dodging file lock issues).  No
> physical or virtual memory need actually be reserved for such a mapping.
>
> --
> - DML
>