patch 9.0.0772: the libvterm code is outdated

Problem:    The libvterm code is outdated.
Solution:   Include libvterm changes from revision 790 to 801.
diff --git a/src/libvterm/README b/src/libvterm/README
index a5b0377..8e9341d 100644
--- a/src/libvterm/README
+++ b/src/libvterm/README
@@ -8,7 +8,7 @@
 
 Modifications:
 - Add a .gitignore file.
-- Convert from C99 to C90.
+- Convert some code from C99 to C90.
 - Other changes to support embedding in Vim.
 
 To get the latest version of libvterm you need the "bzr" command and do: