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.

src/libvterm/include/vterm_keycodes.h

@@ -0,0 +1,58 @@
+#ifndef __VTERM_INPUT_H__
+#define __VTERM_INPUT_H__
+
+typedef enum {
+  VTERM_MOD_NONE  = 0x00,
+  VTERM_MOD_SHIFT = 0x01,
+  VTERM_MOD_ALT   = 0x02,
+  VTERM_MOD_CTRL  = 0x04
+} VTermModifier;
+
+typedef enum {
+  VTERM_KEY_NONE,
+
+  VTERM_KEY_ENTER,
+  VTERM_KEY_TAB,
+  VTERM_KEY_BACKSPACE,
+  VTERM_KEY_ESCAPE,
+
+  VTERM_KEY_UP,
+  VTERM_KEY_DOWN,
+  VTERM_KEY_LEFT,
+  VTERM_KEY_RIGHT,
+
+  VTERM_KEY_INS,
+  VTERM_KEY_DEL,
+  VTERM_KEY_HOME,
+  VTERM_KEY_END,
+  VTERM_KEY_PAGEUP,
+  VTERM_KEY_PAGEDOWN,
+
+  VTERM_KEY_FUNCTION_0   = 256,
+  VTERM_KEY_FUNCTION_MAX = VTERM_KEY_FUNCTION_0 + 255,
+
+  VTERM_KEY_KP_0,
+  VTERM_KEY_KP_1,
+  VTERM_KEY_KP_2,
+  VTERM_KEY_KP_3,
+  VTERM_KEY_KP_4,
+  VTERM_KEY_KP_5,
+  VTERM_KEY_KP_6,
+  VTERM_KEY_KP_7,
+  VTERM_KEY_KP_8,
+  VTERM_KEY_KP_9,
+  VTERM_KEY_KP_MULT,
+  VTERM_KEY_KP_PLUS,
+  VTERM_KEY_KP_COMMA,
+  VTERM_KEY_KP_MINUS,
+  VTERM_KEY_KP_PERIOD,
+  VTERM_KEY_KP_DIVIDE,
+  VTERM_KEY_KP_ENTER,
+  VTERM_KEY_KP_EQUAL,
+
+  VTERM_KEY_MAX /* Must be last */
+} VTermKey;
+
+#define VTERM_KEY_FUNCTION(n) (VTERM_KEY_FUNCTION_0+(n))
+
+#endif