Make {get,set}_inferior_io_terminal inferior methods
This converts the get_inferior_io_terminal and
set_inferior_io_terminal free functions to inferior methods.
Since the related commands are called "tty", "{set,show}
inferior-tty", and MI's "-inferior-tty-{set,show}", to make the
connection between the commands and the code more obvious, the methods
are named set_tty/tty instead of set_io_terminal/io_terminal.