Candlerb/expand xlat

Replaces #186

* argv splitting code factored out as suggested by Alan deKok, so it can be shared by rlm_exec and rlm_redis
* two other fixes to rlm_rediswho

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.