DRTVWR-476: Fix _open_osfhandle() param from long to intptr_t.

The Microsoft _open_osfhandle() opens a HANDLE to produce a C-style int file
descriptor suitable for passing to _fdopen(). We used to cast the HANDLEs
returned by GetStdHandle() to long to pass to _open_osfhandle(). Since HANDLE
is an alias for a pointer, this no longer works.

Fortunately _open_osfhandle() now accepts intptr_t, so we can change the
relevant GetStdHandle() calls. (But why not simply accept HANDLE in the first
place?)