Ability to log to standard output/error

[schildbach]

Currently, opendmarc seems hardcoded to log to /dev/log. This is very problematic for running inside a container, because syslogd won't be available (and can't be run from a user because it needs root).

I think the only thing that's necessary is the ability to log to /dev/stdout as that's where logs are expected by containers.

Optionally, the destination filename could be made configurable.