* progmodes/gdb-mi.el: Fix non-responsive gud commands (bug#13845)

* progmodes/gdb-mi.el: Fix non-responsive gud commands (bug#13845)
(gdb-handler-alist, gdb-handler-number): Remove variables.
(gdb-handler-list): New variable.
(gdb-add-handler, gdb-delete-handler, gdb-get-handler-function)
(gdb-pending-handler-p, gdb-handle-reply)
(gdb-remove-all-pending-triggers): New functions.
(gdb-discard-unordered-replies): New defcustom.
(gdb-handler): New defstruct.
(gdb-wait-for-pending): Fix invalid backquote.  Use gdb-handler-list.
instead of gdb-pending-triggers.  Update docstring.
(gdb-init-1): Remove dead variables.  Initialize gdb-handler-list.
(gdb-speedbar-update, gdb-speedbar-timer-fn, gdb-var-update)
(gdb-var-update-handler, def-gdb-auto-update-trigger)
(def-gdb-auto-update-handler, gdb-get-changed-registers)
(gdb-changed-registers-handler, gdb-get-main-selected-frame)
(gdb-frame-handler): Pending triggers are now automatically managed.
(def-gdb-trigger-and-handler, def-gdb-auto-update-handler):
Remove argument.
(gdb-input): Automatically handles pending triggers.  Update docstring.
(gdb-resync): Replace gdb-pending-triggers by gdb-handler-list.
(gdb-thread-exited, gdb-thread-selected, gdb-register-names-handler):
Update comments.
(gdb-done-or-error): Now use gdb-handle-reply.