journald: wider range of log severities

The previous code made it impossible to distinguish between the `debug`
and `trace` severities. So instead of trying hard to preserve the
equivalence of `spdlog::level::debug` with `LOG_DEBUG`, which is -- in
the end -- just an arbitrary name, make sure that functionality is not
lost.

include/spdlog/sinks/systemd_sink.h

@@ -26,12 +26,12 @@ class systemd_sink : public base_sink<Mutex>
     //
     systemd_sink()
         : syslog_levels_{{/* spdlog::level::trace      */ LOG_DEBUG,
-              /* spdlog::level::debug      */ LOG_DEBUG,
-              /* spdlog::level::info       */ LOG_INFO,
+              /* spdlog::level::debug      */ LOG_INFO,
+              /* spdlog::level::info       */ LOG_NOTICE,
               /* spdlog::level::warn       */ LOG_WARNING,
               /* spdlog::level::err        */ LOG_ERR,
               /* spdlog::level::critical   */ LOG_CRIT,
-              /* spdlog::level::off        */ LOG_INFO}}
+              /* spdlog::level::off        */ LOG_ALERT}}
     {}
 
     ~systemd_sink() override {}