agent: In-review fixes

What have been done:
- the VnswInterfaceListener class has become VnswInterfaceListenerBase
  class (abstract) that is used to deliver FreeBSD and Linux versions
  of this class.
- FreeBSDVnswInterfaceListener class is nearly complete.
- Linux version of VnswInterfaceListener is started.
- Fixes to some sconscripts and not running tests