Make {get,set}_inferior_io_terminal inferior methods

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.

gdb/ChangeLog:

	* fork-child.c (prefork_hook): Adjust.
	* infcmd.c (set_inferior_io_terminal, get_inferior_io_terminal):
	Delete.
	(set_inferior_tty_command, show_inferior_tty_command): Adjust.
	* inferior.c (inferior::set_tty, inferior::tty): New methods.
	* inferior.h (set_inferior_io_terminal, get_inferior_io_terminal):
	Remove declarations.
	(struct inferior) <set_tty, tty>: New methods.
	(struct inferior) <terminal>: Rename to ...
	(struct inferior) <m_terminal>: ... this and make private.
	* main.c (captured_main_1): Adjust.
	* mi/mi-cmd-env.c (mi_cmd_inferior_tty_set): Adjust.
	(mi_cmd_inferior_tty_show): Adjust.
	* nto-procfs.c (nto_procfs_target::create_inferior): Adjust.
	* windows-nat.c (windows_nat_target::create_inferior): Adjust.