(any_kboard_state): Renamed from unlock_kboard.

(any_kboard_state): Renamed from unlock_kboard.
Define it unconditionally, but it's a no-op unless MULTI_KBOARD.
(single_kboard_state): New function.
(push_frame_kboard, pop_frame_kboard): New functions.
(single_kboard): Renamed from kboard_locked.

(unlock_kboard): Do nothing with Vunread_command_events.
(read_char): Clean up logic for reading from KBOARD queues.

(kbd_buffer_get_event): Set copy by assignment, not init.