patch 8.1.0909: MS-Windows: using ConPTY even though it is not stable

Problem:    MS-Windows: using ConPTY even though it is not stable.
Solution:   When ConPTY version is unstable, prefer using winpty. (Ken Takata,
            closes #3949)
diff --git a/src/proto/os_win32.pro b/src/proto/os_win32.pro
index 7f45c5c..6157e01 100644
--- a/src/proto/os_win32.pro
+++ b/src/proto/os_win32.pro
@@ -70,7 +70,9 @@
 void fix_arg_enc(void);
 int mch_setenv(char *var, char *value, int x);
 void control_console_color_rgb(void);
-int has_vtp_working(void);
 int use_vtp(void);
 int is_term_win32(void);
+int has_vtp_working(void);
+int has_conpty_working(void);
+int is_conpty_stable(void);
 /* vim: set ft=c : */