Added sccp_channel_stop_and_deny_scheduled_tasks function, to be called during hangup, to prevent any further scheduled tasks after hangup has been initiated (also used during channel_cleanup, to remove cancelled scheduled tasks)

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5554 43c1dc8c-776e-4df2-8ef1-6e829299be21
Check channel->scheduler.deny before adding new scheduled task

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5555 43c1dc8c-776e-4df2-8ef1-6e829299be21
Check hangup status to cancel softswitch when channel is already being hungup, set sccp device to onhook

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5556 43c1dc8c-776e-4df2-8ef1-6e829299be21
requestHangup should force channel state ONHOOK when called when remote side has already initiate a hangup

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5557 43c1dc8c-776e-4df2-8ef1-6e829299be21