rlm_redis argument splitting for v2.x.x

(#189 rebased)

argv splitting code factored out as suggested by Alan deKok, so it can be shared by rlm_exec and rlm_redis.

Behaviour change: argv elements which consist of quoted strings have the outer quotes stripped off, e.g. `%{exec:/bin/sh -c 'echo -n hello world'}` now works.

Also fix the sample rediswho config file so it doesn't generate deprecation warnings.