Make input_fd be per UI

Make input_fd be per UI

And with that, we can switch the current UI to the UI whose input
descriptor woke up the event loop.  IOW, if the user types in UI 2,
the event loop wakes up, switches to UI 2, and processes the input.
Next the user types in UI 3, the event loop wakes up and switches to
UI 3, etc.

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* event-top.c (input_fd): Delete.
	(stdin_event_handler): Switch to the UI whose input descriptor got
	the event.  Adjust to per-UI input_fd.
	(gdb_setup_readline): Don't set the input_fd global.  Adjust to
	per-UI input_fd.
	(gdb_disable_readline): Adjust to per-UI input_fd.
	* event-top.h (input_fd): Delete declaration.
	* linux-nat.c (linux_nat_terminal_inferior): Don't remove input_fd
	from the event-loop here.
	(linux_nat_terminal_ours): Don't register input_fd in the
	event-loop here.
	* main.c (captured_main): Adjust to per-UI input_fd.
	* remote.c (remote_terminal_inferior): Don't remove input_fd from
	the event-loop here.
	(remote_terminal_ours): Don't register input_fd in the event-loop
	here.
	* target.c: Include top.h and event-top.h.
	(target_terminal_inferior): Remove input_fd from the event-loop
	here.
	(target_terminal_ours): Register input_fd in the event-loop.
	* top.h (struct ui) <input_fd>: New field.