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/terminal.c b/src/terminal.c
index 2544f85..3be2698 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -5521,7 +5521,7 @@
     if (handled)
 	return result;
 
-    if (!has_vtp_working())
+    if (!has_conpty_working())
     {
 	handled = TRUE;
 	result = FAIL;
@@ -6139,7 +6139,7 @@
 
     if (tty_type == NUL)
     {
-	if (has_conpty)
+	if (has_conpty && (is_conpty_stable() || !has_winpty))
 	    use_conpty = TRUE;
 	else if (has_winpty)
 	    use_winpty = TRUE;