ns2: implement block and unblock
Add a vty command to block and unblock a NS-VC.
- ttcn3: test block
- ttcn3: test unblock
- libosmocore: implement vty
gprs_ns2: implement BLOCK/UNBLOCK of a NSVC by vty
The vty should be able to block or unblock a specific NSVC.
Further more this case is special for the UNITDATA as those
can be still received until the other side response to the BLOCK PDU.