patch 8.0.0840: MS-Windows: fopen() and open() prototypes are wrong

Problem:    MS-Windows: fopen() and open() prototypes do not match the ones in
            the system header file.  Can't build without FEAT_MBYTE.
Solution:   Add "const".  Move macro to after including protoo.h.
diff --git a/src/proto/os_win32.pro b/src/proto/os_win32.pro
index 6c5dc4f..754df4f 100644
--- a/src/proto/os_win32.pro
+++ b/src/proto/os_win32.pro
@@ -4,6 +4,8 @@
 void *get_dll_import_func(HINSTANCE hInst, const char *funcname);
 int dyn_libintl_init(void);
 void dyn_libintl_end(void);
+int null_libintl_putenv(const char *envstring);
+int null_libintl_wputenv(const wchar_t *envstring);
 void PlatformId(void);
 void mch_setmouse(int on);
 void mch_update_cursor(void);
@@ -56,8 +58,8 @@
 int mch_rename(const char *pszOldFile, const char *pszNewFile);
 char *default_shell(void);
 int mch_access(char *n, int p);
-int mch_open(char *name, int flags, int mode);
-FILE *mch_fopen(char *name, char *mode);
+int mch_open(const char *name, int flags, int mode);
+FILE *mch_fopen(const char *name, const char *mode);
 int mch_copy_file_attribute(char_u *from, char_u *to);
 int myresetstkoflw(void);
 int get_cmd_argsW(char ***argvp);