patch 8.0.0670: can't use input() in a timer callback

Problem:    Can't use input() in a timer callback. (Cosmin Popescu)
Solution:   Reset vgetc_busy and set timer_busy. (Ozaki Kiichi, closes #1790,
            closes #1129)

src/globals.h

@@ -1659,6 +1659,7 @@ EXTERN int  in_free_unref_items INIT(= FALSE);
 
 #ifdef FEAT_TIMERS
 EXTERN int  did_add_timer INIT(= FALSE);
+EXTERN int  timer_busy INIT(= 0);   /* when timer is inside vgetc() then > 0 */
 #endif
 
 #ifdef FEAT_EVAL