bug#23980: 25.0.95; Events put in `unread-command-events' can be wrongly handled

[Chris Feng <chris.w.feng@gmail.com>]

I encountered a bug in the command loop that when I put an event of the
form (t . EVENT) into `unread-command-events', there is a chance the
event won't get recognized with the message "<t> is undefined".
I did a bit research and found that in keyboard.c `read_char' calls
`read_decoded_event_from_main_queue' (which in turn calls
`read_event_from_main_queue' and `kbd_buffer_get_event') but does not
check if the returned event is from `Vunread_command_events'.
The problem in `read_char' can be demonstrated by the following form:

  (progn
    (run-with-timer 1 nil (lambda () (push '(t . 1) unread-command-events)))
    (read-event nil nil 2))

    => '(t . 1)

There is actually another related issue, that for an event returned by
`read_decoded_event_from_main_queue' other than the form (t . EVENT), we
can't tell whether it's from `Vunread_command_events' or not.
If it is, then we should not put it into `this_command_keys'.

Any ideas on how to fix these?