s390/con3270: add key help to status area

To let the user know about function key bindings, print
them next to the Running/History field at the lower right
of the screen. Also print the scrollback position.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>