add call completion extensions
force cc_agent_policy to never on forward

What happens if it is set to generic is that the caller could then
request call completion, but it might not work as he expected.

An example is a user A with a schedule that is calling user B; if
someone call user A, user B will ring. If the caller then request call
completion, it will monitor B's phone, but when B becomes available, it
will dial user A extension.
use a single "toggle" extension for call completion

Instead of an extension to request it and another to cancel it.
set CC_EXTEN and CC_CONTEXT
Merge branch '5141-ccss'