rlm_redis argument splitting for master

Same functionality as #192 for master branch

argv splitting code factored out, shared between rlm_exec and rlm_redis. Makes rlm_redis able to cope with keys and values which contain spaces.

Behaviour change for rlm_exec: quoted string literals have the outer quotes stripped off in argv array, e.g. %{exec:/bin/sh -c 'echo -n hello world'}