patch 8.0.1647: terminal API may call any user function

Problem:    Terminal API may call a function not meant to be called by this
            API.
Solution:   Require the function to start with Tapi_.
diff --git a/src/terminal.c b/src/terminal.c
index 50e87c9..5875af6 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -3193,7 +3193,7 @@
     }
     func = get_tv_string(&item->li_tv);
 
-    if (!ASCII_ISUPPER(*func))
+    if (STRNCMP(func, "Tapi_", 5) != 0)
     {
 	ch_log(channel, "Invalid function name: %s", func);
 	return;