snmpit_apcon.pm: Add emptyVLAN warning support when unloading snmpit_apcon package.
snmpit_apcon.pm: Add emptyVLAN warning support when unloading snmpit_apcon package.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome