- Add --with-libhiredis, unbound support for a new cached backend

- Add --with-libhiredis, unbound support for a new cached backend
  that uses a Redis server as the storage.  This implementation
  depends on the hiredis client library (https://redislabs.com/lp/hiredis/).
  And unbound should be built with both --enable-cachedb and
  --with-libhiredis[=PATH] (where $PATH/include/hiredis/hiredis.h
  should exist).  Patch from Jinmei Tatuya (Infoblox).

git-svn-id: file:///svn/unbound/trunk@4586 be551aaa-1e26-0410-a405-d3ace91eadb9