main: Refactor normal_enter to call os_inchar directly

This makes it impossible for K_EVENT to interfere with mappings, but it also
disables processing of events while in the middle of a mapping (Though this will
be fixed later as this refactoring progresses).

`may_sync_undo` is now called when K_EVENT is received. This is necessary to
correctly update undo entry lists before executing some action.

src/nvim/getchar.c

@@ -1131,7 +1131,7 @@ static void gotchars(char_u *chars, int len)
  * - While reading a script file.
  * - When no_u_sync is non-zero.
  */
-static void may_sync_undo(void)
+void may_sync_undo(void)
 {
   if ((!(State & (INSERT + CMDLINE)) || arrow_used)
       && scriptin[curscript] == NULL)