o consolidate "unbound_" prefix for unbound.inc for pluginification
o services.inc includes unbound.inc rightfully, a bit like a
  plugin would do in the future, but this also means we can
  zap all requires from top pages, which are entirely spurious.
o get_dns_servers() moves to system.inc because it is used by
  status_interfaces.php
o RIP read_hosts()

(cherry picked from commit 715b6367)
(cherry picked from commit cdb0c667)