Fix implementation of the --terminal command-line switch

Fix implementation of the --terminal command-line switch

It sounds like this has been broken ever since multi-tty was
added to Emacs.
* src/keyboard.c (dev_tty): New global variable.
* src/keyboard.h: Declare 'dev_tty'.
* src/emacs.c (main): Initialize 'dev_tty'.
* src/term.c (Fcontrolling_tty_p, Fresume_tty, init_tty):
* src/process.c (dissociate_controlling_tty):
* src/keyboard.c (handle_interrupt_signal, handle_interrupt)
(Fset_quit_char): Use 'dev_tty' instead of 'DEV_TTY'.  (Bug#70519)