allow SIGINT to exit
Fix "malformed format string" syslog error.

The `Syslog.log` method interprets the second argument as format string.
If a log message contains a "%" character (like mongodump output), an
`ArgumentError` gets raised.

This patch converts the `log` call to use a "%s" format string as second
argument and pass the line as third argument.
Release v3.7.1