agetty: update man page and usage()

 - baud_rate is optional
 - agetty has been rewritten 2 years ago, so don't blame original
   authors in the man page

Signed-off-by: Karel Zak <kzak@redhat.com>
agetty: don't remove ECHOCTL from c_lflag

It's really unexpected that arrows keys move cursor when agetty/login
asks for login name or password.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=969458
Signed-off-by: Karel Zak <kzak@redhat.com>