DIMSE_createFilestream() now uses an OFFilename.

The low-level network function DIMSE_createFilestream() now uses an OFFilename
parameter instead of a char*. This allows for passing a wide character string
to this function on Windows. The API remains stable since the OFFilename
constructor also accepts a char*.
Fixed various typo.
Pass OFString to DIMSE_createFilestream().

Pass an OFString filename to DIMSE_createFilestream() instead of char* since
this function now accepts an OFFilename instance.