Clean up read_key_sequence a bit; reread active keymaps after first event.

Clean up read_key_sequence a bit; reread active keymaps after first event.
* src/keyboard.c (read_char, read_char_x_menu_prompt)
(read_char_minibuf_menu_prompt):
Replace nmaps+maps with a single `map' arg.
(follow_key): Operate on a single map.
(active_maps): New function.
(test_undefined): Also return true for nil bindings.
(read_key_sequence): Use active_maps to replace the arrays of keymaps with
a single (composed) keymap.  Remember `first_event' to choose the right
set of active keymaps.  Recompute the set of keymaps after receiving
the first event.  Remove GOBBLE_FIRST_EVENT.
(syms_of_keyboard): Remove inhibit_local_menu_bar_menus.
* src/keyboard.h (read_char): Update declaration.
* src/lread.c (read_filtered_event): Adjust call to read_char.
* lisp/cus-start.el (all): Remove inhibit-local-menu-bar-menus.