Cleanup terminal handling code.

Cleanup terminal handling code.
* dispextern.h (get_named_tty): Remove prototype but...
* termhooks.h (get_named_terminal): ...resurrect it under
more meaningful name.
(get_terminal): Likewise, but with...
(decode_live_terminal): ...this name.
(decode_tty_terminal): Add prototype.
* term.c (get_tty_terminal): Remove.
(get_named_tty): Remove.
(Ftty_display_color_p, Ftty_display_color_cells, Ftty_type)
(Fcontrolling_tty_p, Fsuspend_tty, Fresume_tty):
Use decode_tty_terminal.
(Ftty_no_underline, Ftty_top_frame): Use decode_live_terminal.
* terminal.c (get_terminal): Refactor to...
(decode_terminal, decode_live_terminal): ...new functions.
(decode_tty_terminal): Replacement for get_tty_terminal.
(get_named_terminal): Likewise for get_named_tty.
* coding.c (Fset_terminal_coding_system_internal)
(Fterminal_coding_system, Fset_keyboard_coding_system_internal):
(Fkeyboard_coding_system):
* composite.c (Fcomposition_get_gstring):
* dispnew.c (Fsend_string_to_terminal):
* frame.c (Fmake_terminal_frame):
* nsfns.m (check_ns_display_info):
* w32fns.c, xfns.c (check_x_display_info):
* xselect.c (frame_for_x_selection): Use decode_live_terminal.
* keyboard.c (handle_interrupt_signal, handle_interrupt)
(Fset_quit_char): Use get_named_terminal.
(Fset_output_flow_control, Fset_input_meta_mode):
Use decode_tty_terminal.