patch 8.0.0693: no terminal emulator support

Problem:    No terminal emulator support.  Cannot properly run commands in the
            GUI.  Cannot run a job interactively with an ssh connection.
Solution:   Very early implementation of the :terminal command.  Includes
            libvterm converted to ANSI C.  Many parts still missing.
diff --git a/src/libvterm/doc/URLs b/src/libvterm/doc/URLs
new file mode 100644
index 0000000..3a6512f
--- /dev/null
+++ b/src/libvterm/doc/URLs
@@ -0,0 +1,11 @@
+ECMA-48:
+  http://www.ecma-international.org/publications/standards/Ecma-048.htm
+
+Xterm Control Sequences:
+  http://invisible-island.net/xterm/ctlseqs/ctlseqs.html
+
+Digital VT100 User Guide:
+  http://vt100.net/docs/vt100-ug/
+
+Digital VT220 Programmer Reference Manual
+  http://vt100.net/docs/vt220-rm/