Deal gracefully with up-events (Bug#19746)

Deal gracefully with up-events (Bug#19746)

* keyboard.c (apply_modifiers_uncached, parse_solitary_modifier)
(parse_modifiers_uncached): React gracefully to "up-" modifiers:
those may easily be injected by user-level Lisp code.
(read_key_sequence): Discard unbound up-events like unbound
down-events: they are even more likely only relevant for special
purposes.

While Emacs will not produce up-events on its own currently (those are
converted to drag or click events before being converted to
Lisp-readable structures), the input queue can be made to contain them
by synthesizing events to `unread-command-events'.  Emacs should deal
consistently with such events.