login: use TCSANOW to remove HUPCL

Fedora 19, kernel 3.9, systemd 204, telnetd with socket activation.

login(1) code:

        ttt.c_cflag &= ~HUPCL;
        tcsetattr(0, TCSAFLUSH, &ttt);

        close(STDIN_FILENO);
        close(STDOUT_FILENO);
        close(STDERR_FILENO);

        signal(SIGHUP, SIG_IGN);        /* so vhangup() wont kill us */
        vhangup();
        signal(SIGHUP, SIG_DFL);

strace :

	ioctl(0, SNDCTL_TMR_CONTINUE or SNDRV_TIMER_IOCTL_GPARAMS or TCSETSF, {B9600 opost isig icanon echo ...}) = 0
	close(0)                                = 0
	close(1)                                = 0
	close(2)                                = 0
	--- SIGHUP {si_signo=SIGHUP, si_code=SI_KERNEL} ---
	--- SIGTERM {si_signo=SIGTERM, si_code=SI_USER, si_pid=1, si_uid=0} ---
	+++ killed by SIGTERM +++

It seems we get SIGHUP (and SIGTERM from systemd) before vhangup().
The problem occur sometimes.

I guess it's because TCSAFLUSH is not applied immediately, so it would
be probably better to use TCSANOW to make the code more robust.

References: https://bugzilla.redhat.com/show_bug.cgi?id=962145
Signed-off-by: Karel Zak <kzak@redhat.com>
runuser: update man page, add note about -l vs. -p

Reported-by: Bernhard Voelker <mail@bernhard-voelker.de>
Signed-off-by: Karel Zak <kzak@redhat.com>