Add check for global filtering levels in Log::enabled() impl

[inejge]

The check mimics the ones used by the log!() macro.

The enabled() function is used by the log_enabled!() macro. I stumbled over this when activating logging for a hyper server, which uses its own tracing framework. If enabled() always returns true, hyper will happily spam syslog with trace-level messages despite the filtering having been initialized in syslog::init_unix() or equivalent.

[Geal]

nice catch, thanks