End 2 end CANCEL Reason (and other additional CANCEL headers)

[ThomasSevestre]

This is a solution to #164

Please, be aware...

I've not tested the PR on master but it is working on an older version of sems.
I've used this with SBC application only.
It would be better to pass Reason header only.

[juha-h]

Is relaying of CANCEL headers configurable in sbc profile?

[ThomasSevestre]

Not in this PR.

First it could be a good idea to make sure this is triggered only in SBC module, we need to override onCancel in CallLeg (see here)

This looks simple, I can do it.

CallLeg does not have direct access to profiles, it requires more work to make this configurable. Relaying CANCEL Reason header looks like something that should be done by default.