Fix dnsmasq host overrides and dhcp integration

. Do not execute following actions when dnsmasq is disabled:
  . Add host overrides to /etc/hosts
  . Register DHCP leases in DNS Forwarder
  . Register DHCP static mappings in DNS forwarder

It should fix issue reported at following forum post:

http://forum.pfsense.org/index.php?topic=58871