qemu: create tty5 and tty6 for getties
qemu: create tty5 and tty6 for getties

In addition to interactive askfirst shells on tty2 to tty4, Busybox init
also spwans two getties on tty5 and tty6 within its inittab example.

	# /sbin/getty invocations for selected ttys
	tty5::respawn:/sbin/getty 38400 tty5
	tty6::respawn:/sbin/getty 38400 tty6

Let /etc/init.d/rcS makes those two extra tty devices.

But there is still no login accesses with those two pseudo tty using
ctrl-alt-f5 and ctrl-alt-f6. Getty reports:

	getty: can't open '/dev/null': No such file or directory
	getty: can't open '/dev/null': No such file or directory
	getty: can't open '/dev/null': No such file or directory
	...

Getty opens /dev/null device. Thus, let /etc/init.d/rcS also makes the
null device.

Note 1: here is a manual test:

	# /bin/mknod -m 666 /dev/null c 1 3
	# true | /sbin/getty 38400 tty5
	process 28 (getty) attempted a POSIX timer syscall while CONFIG_POSIX_TIMERS is not set

Then ctrl+alt+2, sendkey ctrl-alt-f5, ctrl+alt+1, and the pseudo
terminal tty5 shows:

	(none) login: 1

Note 2: when getty is executed manually, it complains about:

	# /sbin/getty 38400 tty5
	getty: setsid: Operation not permitted

A comment in the code of Busybox tells:

	bb_perror_msg_and_die("setsid");
	/*
	 * When we can end up here?
	 * Example: setsid() fails when run alone in interactive shell:
	 *  # getty 115200 /dev/tty2
	 * because shell's child (getty) is put in a new process group.
	 * But doesn't fail if shell is not interactive
	 * (and therefore doesn't create process groups for pipes),
	 * or if getty is not the first process in the process group:
	 *  # true | getty 115200 /dev/tty2
	 */

Getty have to be piped when is it run from an interactive shell.

	/ # true | /sbin/getty 38400 tty5
	getty: can't open '/dev/null': No such file or directory

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