Provides a cleaner split between log sources, specifically for not
compiling with systemd libraries.
This is in preparation for a new log source to read from Docker.
The following flags configure this usage:
- systemd.enable: enable collection from systemd.
- systemd.unit: systemd unit, postfix.service by default.
- systemd.slice: systemd slice, overrides the unit.
- systemd.journal_path: path to the systemd journal directory.
Closes#2