Fix GCC compile warning for "bad-function-cast" on Windows

Fix GCC compile warning for "bad-function-cast" on Windows

Trick the compiler by hiding the cast in a function call.

* src/os.h: Declare the new function.
* src/w32/w32os.c (get_handle_for_fd): Convert and cast a file
descriptor into a Windows HANDLE.
(check_io_state): Call the new function rather than casting.
(osync_release): Ditto.
(fd_inherit): Ditto.
(fd_noinherit): Ditto.
* src/function.c (windows32_openpipe): Ditto.
* src/w32/compat/posixfcn.c (isatty): Ditto.
* src/w32/subproc/sub_proc.c (process_easy): Ditto.