As `SyslogLogHandler` is a value type it isn't necessary to lock access
to its properties.

`openlog()` may retain the `ident` internally, so we can't pass it the
label's raw buffer.  Also, it's not necessary to call `openlog()`
multiple times which would happen when it's called from the
`LogHandler`'s initialization.

Instead, pass the logger label in the log message.