win32,python3: Fix crash when stdin is redirected

This causes crash on Windows when Vim is linked with Python 3.5 or later:
```
echo 1 | vim -c "py3 print(1)" -
```

Python 3.5 or later calls `io.py` in `Py_Initialize()`, and it calls
`abort()` when stdin is redirected. This can be avoided by reconnecting
stdin to CONIN$.

However, we should consider that the python DLL and the Vim executable
might be linked to different C runtime libraries. This tries to get
stdin and freopen which are actually used in the python DLL, then calls
`freopen("CONIN$", "r", stdin)` to reconnect stdin.

src/Make_cyg_ming.mak

@@ -588,6 +588,8 @@ ifdef PYTHON3
 CFLAGS += -DFEAT_PYTHON3
  ifeq (yes, $(DYNAMIC_PYTHON3))
 CFLAGS += -DDYNAMIC_PYTHON3 -DDYNAMIC_PYTHON3_DLL=\"$(DYNAMIC_PYTHON3_DLL)\"
+ else
+CFLAGS += -DPYTHON3_DLL=\"$(DYNAMIC_PYTHON3_DLL)\"
  endif
 endif
 

src/Make_mvc.mak

@@ -1026,6 +1026,9 @@ PYTHON_LIB = $(PYTHON)\libs\python$(PYTHON_VER).lib
 ! ifndef PYTHON3_VER
 PYTHON3_VER = 36
 ! endif
+! ifndef DYNAMIC_PYTHON3_DLL
+DYNAMIC_PYTHON3_DLL = python$(PYTHON3_VER).dll
+! endif
 ! message Python3 requested (version $(PYTHON3_VER)) - root dir is "$(PYTHON3)"
 ! if "$(DYNAMIC_PYTHON3)" == "yes"
 !  message Python3 DLL will be loaded dynamically
@@ -1035,9 +1038,10 @@ PYTHON3_OBJ = $(OUTDIR)\if_python3.obj
 PYTHON3_INC = /I "$(PYTHON3)\Include" /I "$(PYTHON3)\PC"
 ! if "$(DYNAMIC_PYTHON3)" == "yes"
 CFLAGS = $(CFLAGS) -DDYNAMIC_PYTHON3 \
-		-DDYNAMIC_PYTHON3_DLL=\"python$(PYTHON3_VER).dll\"
+		-DDYNAMIC_PYTHON3_DLL=\"$(DYNAMIC_PYTHON3_DLL)\"
 PYTHON3_LIB = /nodefaultlib:python$(PYTHON3_VER).lib
 ! else
+CFLAGS = $(CFLAGS) -DPYTHON3_DLL=\"$(DYNAMIC_PYTHON3_DLL)\"
 PYTHON3_LIB = $(PYTHON3)\libs\python$(PYTHON3_VER).lib
 ! endif
 !endif

src/if_python3.c

@@ -907,6 +907,47 @@ python3_loaded(void)
 
 static wchar_t *py_home_buf = NULL;
 
+#if defined(MSWIN) && (PY_VERSION_HEX >= 0x030500f0)
+// Python 3.5 or later will abort inside Py_Initialize() when stdin is
+// redirected.  Reconnect stdin to CONIN$.
+// Note that the python DLL is linked to its own stdio DLL which can be
+// differ from Vim's stdio.
+    static void
+reset_stdin(void)
+{
+    FILE *(*py__acrt_iob_func)(unsigned) = NULL;
+    FILE *(*pyfreopen)(const char *, const char *, FILE *) = NULL;
+    HINSTANCE hinst;
+
+# ifdef DYNAMIC_PYTHON3
+    hinst = hinstPy3;
+# else
+    hinst = GetModuleHandle(PYTHON3_DLL);
+# endif
+    if (hinst == NULL)
+	return;
+
+    // Get "freopen" and "stdin" which are used in the python DLL.
+    // "stdin" is defined as "__acrt_iob_func(0)" in VC++ 2015 or later.
+    py__acrt_iob_func = get_dll_import_func(hinst, "__acrt_iob_func");
+    if (py__acrt_iob_func)
+    {
+	HINSTANCE hpystdiodll = find_imported_module_by_funcname(hinst,
+							"__acrt_iob_func");
+	if (hpystdiodll)
+	    pyfreopen = (void*)GetProcAddress(hpystdiodll, "freopen");
+    }
+
+    // Reconnect stdin to CONIN$.
+    if (pyfreopen)
+	pyfreopen("CONIN$", "r", py__acrt_iob_func(0));
+    else
+	freopen("CONIN$", "r", stdin);
+}
+#else
+# define reset_stdin()
+#endif
+
     static int
 Python3_Init(void)
 {
@@ -939,6 +980,7 @@ Python3_Init(void)
 
 	PyImport_AppendInittab("vim", Py3Init_vim);
 
+	reset_stdin();
 	Py_Initialize();
 
 	// Initialise threads, and below save the state using