Allow configuring options to serial console getty.

Add parameter 'options' to the console::serial::login definition,
allowing users to change which options get sent to the agetty process.

The default is intended to be the same as what the operating system
normally does; under systemd we thus pass --keep-baud by default, but
on RHEL-6 we default to no options.  (This is also the same behaviour
as we had before, so users that don't set the 'options' parameter will
not get any changes.)

Signed-off-by: Thomas Bellman <bellman@lysator.liu.se>