Add auth-only option and sasl alternate auth file for init.d service

In order to run cloudprint as a full init.d service, it would be useful to be able to specify an alternate authentication file that supports the sasl format, and to be able to separate out establishment of authentication credentials from normal service operation. This patch set does that by establishing sasl authentication in the alternate auth file if existing credentials fail, and by adding a '-c' 'establish credentials only' option.

A similar patch is used in [cloudprint-debian](http://davesteele.github.io/cloudprint-debian/). ([man page](http://davesteele.github.io/cloudprint-debian/cloudprint-service.8.html)).