Outgoing BRI calls fail when using Asterisk 1.8 with HA8, HB8, and B410P cards.

Outgoing BRI calls fail when using Asterisk 1.8 with HA8, HB8, and B410P cards.

France Telecom brings layer 2 and layer 1 down on BRI lines when the line
is idle.  When layer 1 goes down Asterisk cannot make outgoing calls and
the HA8 and HB8 cards also get IRQ misses.

The inability to make outgoing calls is because the line is in red alarm
and Asterisk will not make calls over a line it considers unavailable.
The IRQ misses for the HA8 and HB8 card are because the hardware is
switching clock sources from the line which just brought layer 1 down to
internal timing.

There is a DAHDI option for the B410P card to not tell Asterisk that layer
1 went down so Asterisk will allow outgoing calls: "modprobe wcb4xxp
teignored=1".  There is a similar DAHDI option for the HA8 and HB8 cards:
"modprobe wctdm24xxp bri_teignored=1".  Unfortunately that will not clear
up the IRQ misses when the telco brings layer 1 down.

* Add layer 2 persistence option to customize the layer 2 behavior on BRI
PTMP lines.  The new option has three settings: 1) Use libpri default
layer 2 setting.  2) Keep layer 2 up.  Bring layer 2 back up when the peer
brings it down.  3) Leave layer 2 down when the peer brings it down.
Layer 2 will be brought up as needed for outgoing calls.

JIRA AST-598

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@332264 65c4cc65-6c06-0410-ace0-fbb531ad65f3