fix output redirection of 'anything' resource

This fixes #496 by making the `logfile` parameter default to `/dev/null`.

This has the added benefit of also fixing potential SIGPIPE errors. Previously, if no `logfile` parameter were provided, once the shell that started the process stops listening for output (when [line 85](https://github.com/ClusterLabs/resource-agents/blob/d1b1ed9c9793bdf384c72553267ee3e1f66f555f/heartbeat/anything#L85) ends), if the process tries to write to STDOUT, it'll get a SIGPIPE as there's no longer anything listening on the other end.