patch 8.1.0124: has('vcon') returns true even for non-win32 terminal

Problem:    has('vcon') returns true even for non-win32 terminal.
Solution:   Check the terminal type. (Nobuhiro Takasaki, closes #3106)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index cec6dab..ce45ad8 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -6609,7 +6609,7 @@
 #endif
 #ifdef FEAT_VTP
 	else if (STRICMP(name, "vcon") == 0)
-	    n = has_vtp_working();
+	    n = is_term_win32() && has_vtp_working();
 #endif
 #ifdef FEAT_NETBEANS_INTG
 	else if (STRICMP(name, "netbeans_enabled") == 0)