- creates proper ifc.h and dhcp.h headers for libnetutils

- adds ifc_set_hwaddr
- adds hwaddr command to netcfg
- code reuse: dhcp_configure calls ifc_configure; inet_ntoa is used for printing
- consistency: use net.XXX.dnsX properties in favor of dhcp.XXX.dnsX properties
  (see related change to WifiStateTracker)
- updated system/core/nexus to use new headers, although not sure if
anybody still uses nexus

Change-Id: Idd70c0ac6e89b38e86816578c33eff805d30cac4