agent: Preparing VnswInterfaceListenerLinux

What have been done:
 - most of linux code has been created and moved to
   linux_vnswif_listener.cc/h
 - compilation of Linux/FreeBSD has been added to SConscript

What needs to be done:
 - Linux version of SyncCurrentState and IsIfUp are missing
 - lot of commented out code to be rmoved