Generalized logic for setting up /etc/resolv.conf and /etc/hosts

Generalized logic for setting up /etc/resolv.conf and /etc/hosts
depending on whether myplc has DNS enabled or not.

If PLC's DNS (dnsmasq) is enabled, then:

1) /etc/resolv.conf will point at the local host and
   /etc/plc_resolv.conf points either at:

  a) the plc-config-tty configured DNS servers, or
  b) the servers specific in the original /etc/resolv.conf

2) /etc/plc_hosts will contain a list of the currently configured
   nodes. (This needs to be fixed, as the set of nodes and their
   network settings can change).

If PLC's DNS (dnsmasq) is disabled, then /etc/resolv.conf will either :
  a) remain untouched if PLC_NET_DNS{1,2} are not specified, or
  b) be backed up to /etc/resolv_plc.backup and a new /etc/resolv.conf
     will be generated with the PLC_NET_DNS{1,2} servers configured by
     plc-config-tty