Use win32 DuplicatHandle instead of _dup

There are constraints when using the posix API on win32
that aren't present with normal win32.

Author: bakpakin

src/core/ev.c

@@ -3490,7 +3490,7 @@ JANET_CORE_FN(janet_cfun_ev_all_tasks,
 JANET_CORE_FN(janet_cfun_to_stream,
               "(ev/to-stream file)",
               "Convert a core/file to a core/stream. On POSIX operating systems, this will mark "
-              "the underlying open file description as non-blocking.") {
+              "the underlying open file descriptor as non-blocking.") {
     janet_fixarity(argc, 1);
     int32_t flags = 0;
     int32_t stream_flags = 0;
@@ -3500,10 +3500,13 @@ JANET_CORE_FN(janet_cfun_to_stream,
     if (flags & JANET_FILE_NOT_CLOSEABLE) stream_flags |= JANET_STREAM_NOT_CLOSEABLE;
     if (flags & JANET_FILE_CLOSED) janet_panic("file is closed");
 #ifdef JANET_WINDOWS
-    int fno = _fileno(file);
-    int dupped_fno = _dup(fno);
-    if (dupped_fno == -1) janet_panic(janet_strerror(errno));
-    JanetStream *stream = janet_stream(_get_osfhandle(dupped_fno), stream_flags, NULL);
+    HANDLE handle = (HANDLE) _get_osfhandle(_fileno(file));
+    HANDLE prochandle = GetCurrentProcess();
+    HANDLE dupped_handle = INVALID_HANDLE_VALUE;
+    if (!DuplicateHandle(prochandle, handle, prochandle, &dupped_handle, 0, FALSE, DUPLICATE_SAME_ACCESS)) {
+        janet_panic("cannot duplicate handle to file");
+    }
+    JanetStream *stream = janet_stream(dupped_handle, stream_flags, NULL);
 #else
     int handle = fileno(file);
     int dupped_handle = 0;

src/core/os.c

@@ -1254,15 +1254,15 @@ static Janet os_execute_impl(int32_t argc, Janet *argv, JanetExecuteMode mode) {
     } else if (new_in != JANET_HANDLE_NONE) {
         startupInfo.hStdInput = new_in;
     } else {
-        startupInfo.hStdInput = (HANDLE) _get_osfhandle(0);
+        startupInfo.hStdInput = (HANDLE) _get_osfhandle(_fileno(stdin));
     }
 
     if (pipe_out != JANET_HANDLE_NONE) {
         startupInfo.hStdOutput = pipe_out;
     } else if (new_out != JANET_HANDLE_NONE) {
         startupInfo.hStdOutput = new_out;
     } else {
-        startupInfo.hStdOutput = (HANDLE) _get_osfhandle(1);
+        startupInfo.hStdOutput = (HANDLE) _get_osfhandle(_fileno(stdout));
     }
 
     if (pipe_err != JANET_HANDLE_NONE) {
@@ -1272,7 +1272,7 @@ static Janet os_execute_impl(int32_t argc, Janet *argv, JanetExecuteMode mode) {
     } else if (stderr_is_stdout) {
         startupInfo.hStdError = startupInfo.hStdOutput;
     } else {
-        startupInfo.hStdError = (HANDLE) _get_osfhandle(2);
+        startupInfo.hStdError = (HANDLE) _get_osfhandle(_fileno(stderr));
     }
 
     int cp_failed = 0;