Previously this was in __dahdi_init_span(). The problem was that
__dahdi_init_span() was only called when a spans' line mode was being changed.
Therefore it was possible to unassign and resassign an analog span and leave it
stuck in the 'NOP' alarm state.
It also make the setting / clearing of DAHDI_ALARM_NOTOPEN symetrical about span
unassignment / assignment in addition to updating the alarm states on all the
channels on the span via the dahdi_alarm_notify() function.
This is a better version of commit 496f817773.
Signed-off-by: Shaun Ruffell <sruffell@digium.com>