rlm_rediswho: fix trim functionality

(note: query result is not available after redis_finish_query has been called)
rlm_rediswho: return RLM_MODULE_FAIL on failure (including failed Expire)
rlm_rediswho: allow insert/trim/expire to be configured independently

For example you might want to do SET and EXPIRE on a start record,
and just EXPIRE on interim and stop.
Remove unnecessary variable
rlm_rediswho: fix unlang conditional expansion