patch 8.0.0740: cannot resize a terminal window by the command

Problem:    Cannot resize a terminal window by the command running in it.
Solution:   Add support for the window size escape sequence.  Make BS work.
diff --git a/src/terminal.c b/src/terminal.c
index ac5b7b8..4069e9b 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -572,7 +572,8 @@
     {
 	case CAR:		key = VTERM_KEY_ENTER; break;
 	case ESC:		key = VTERM_KEY_ESCAPE; break;
-	case K_BS:		key = VTERM_KEY_BACKSPACE; break;
+				/* VTERM_KEY_BACKSPACE becomes 0x7f DEL */
+	case K_BS:		c = BS; break;
 	case K_DEL:		key = VTERM_KEY_DEL; break;
 	case K_DOWN:		key = VTERM_KEY_DOWN; break;
 	case K_END:		key = VTERM_KEY_END; break;