Rotating file logger

[maximevanhees]

The logs of services can now also be written to file by using the log: file and log_file: log_filename.txt options in the <service>.yaml file.

Example: /etc/zinit/example.yaml:

exec: sh -c "echo 'writing example log line'"
log: file
log_file: "log.txt"

This will create a log.txt file in the /var/log/zinit/<service_name>/ directory. At this moment a log file can be at max 1MiB in size. If the size of the log exceeds this threshold, the file is moved to log_<datetime>.txt file. There can be a maximum of 5 rotation files, this to keep the disk usage to a minimum.

Note: even when using the log: file option, the logs from the service can still be accessed through the zinit log command as well.

Closes #68.