Implement a global switch on the network to disable call waiting.

Add a network -> callwaiting VTY command as boolean.

When this is enabled (default) there is no change to
operation previous to this commit.

When this switch is disabled with "no call-waiting" in vty
then when a call arrives, we will check if we have an active
call transaction for this subscriber, no matter if it is
establishing, established, or alerting, in any of these cases we
will return USER BUSY to the calling party.

Change-Id: I3eb6f23f7103e3002874fb5d3a30c9de952202ae
5 files changed