datapaths: Allow pcap_switch without pxpcap
datapaths: Allow pcap_switch without pxpcap

pcap_switch has virtual ports which don't require pxpcap, so we no
longer strictly require pxpcap.

MurphyMc/pox,noxrepo/pox,noxrepo/pox,noxrepo/pox,MurphyMc/pox,noxrepo/pox,MurphyMc/pox,MurphyMc/pox,MurphyMc/pox