FDI: Constify some variables

[Andrew Talbot]

James Hawkins wrote:

> Are you checking with the SDK to make sure these changes are legit?
> This isn't how the Windows SDK fdi.h has these defined.
> 

I ascertained that the function-pointer parameters of FDICreate() should
have the same signatures as the corresponding low-level file functions
(e.g., pfnopen should point to a function that takes and returns identical
parameters to _open(), pfnwrite to _write(). Its pfnopen, is of type
PFNOPEN and its pfnwrite of type PFNWRITE, so I constified those types to
match the signatures of _open() and _write(), respectively, and altered the
functions that were affected.

-- Andy.