Receive an Application Information Request from the BTS via PCU
interface. Construct a Packet Application Information message from it
(3GPP TS 44.060 11.2.47) and send it to all MS with active TBF.
The TTCN-3 test infrastructure to test this feature is not quite ready
yet, so I've added C unit tests instead.
Related: OS#4048
Change-Id: Ie35959f833f46bde5f2126314b6f96763f863b36
