agetty: better support of the special system consoles on S390

agetty: better support of the special system consoles on S390

That is that there are several consoles, the /dev/ttyS0 which
is type of ibm3215 and a dumb terminal, then there is the device
/dev/3270/tty1 which can handle ANSI color escape sequences and is
a ibm327x terminal, and the /dev/ttyS1 which is a vt220 terminal.

The macro is_speed() in agetty.c allows to distinguish between the
terminal line (/dev)3270/tty1 and the speed options on the command
line used in

/run/systemd/generator/getty.target.wants/serial-getty@3270-tty1.service

which is a symbolic link to /usr/lib/systemd/system/serial-getty@.service

Signed-off-by: Werner Fink <werner@suse.de>