Add support for logging into an arbitrary file

In addition to logging to STDOUT and syslog, it would be nice if mylvmbackup could also log messages into an arbitrary log file, as directed by the log_method="file" and log_filename=<filename>

Note that log_method="both" is ambigous in this case and should be replaced with a list, e.g. log_method="syslog,file" or "console"