node: Implement prefix 'unreg' when interest filter is unset

Filter unsetting is useful in some applications that want to terminate
after data has been successfully pushed out to the forwarding daemon.

Change-Id: Iabada0db7d1bd653899941a294abd4c8cd20a80f
tests: Porting forwarding entry test and fixing app tests

Change-Id: Ia6d2a0b39a1105c0873f873bbd4fce49dabb3cca
transport/unix: Fixing segfault due to errorneous creation of multiple UnixTransport::Impl instances

The bug showed itself when multiple expressInterest's were issued before
running processEvents.

Change-Id: I7e60d0ae625777e06471aca00707d7b88e68a5c8