Tie all messages in a chain into the same trail.

Store the trail in the AsChain; retrieve it on the message
that retrieves the chain.
Don't require auth on trusted port.
Make UTs compile.
Add UT for port-dependent auth function.

Allow specifying the transport when injecting a message direct
to one module.
Merge pull request #25 from Metaswitch/urgent-isc-fixes

[Reviewer: Mike] Urgent ISC fixes