patch 9.0.0774: the libvterm code is outdated

Problem:    The libvterm code is outdated.
Solution:   Include libvterm changes from revision 802 to 817.  Revert some
            changes made for C89.
diff --git a/src/libvterm/CONTRIBUTING b/src/libvterm/CONTRIBUTING
index 2100d1e..e9a8f0c 100644
--- a/src/libvterm/CONTRIBUTING
+++ b/src/libvterm/CONTRIBUTING
@@ -6,8 +6,8 @@
   Launchpad
     https://launchpad.net/libvterm
 
-  Freenode:
-    ##tty or #tickit on irc.freenode.net
+  IRC:
+    ##tty or #tickit on irc.libera.chat
 
   Email:
     Paul "LeoNerd" Evans <leonerd@leonerd.org.uk>