journald: structured output for logger's name

Previously, the logger name was effectively lost. There were two choices
on how to add it:

- Via a formatter, which would mean that `journalctl` would not be able
to filter against that. That would be suboptimal.

- As a "syslog identifier". This means that `journalctl` will, by
default, stop showing the daemon's executable name and replace that via
the logger name. The PID is still shown, and if one would like to go
back to the previous behavior, it is still possible via `journalctl -o
with-unit`.

I think that the second option is strictly better than the first one.

fixes #1289

include/spdlog/sinks/systemd_sink.h

@@ -59,11 +59,13 @@ class systemd_sink : public base_sink<Mutex>
         {
             // Note: function call inside '()' to avoid macro expansion
             err = (sd_journal_send)(
-                "MESSAGE=%.*s", static_cast<int>(length), msg.payload.data(), "PRIORITY=%d", syslog_level(msg.level), nullptr);
+                "MESSAGE=%.*s", static_cast<int>(length), msg.payload.data(), "PRIORITY=%d", syslog_level(msg.level),
+                "SYSLOG_IDENTIFIER=%.*s", static_cast<int>(msg.logger_name.size()), msg.logger_name.data(), nullptr);
         }
         else
         {
             err = (sd_journal_send)("MESSAGE=%.*s", static_cast<int>(length), msg.payload.data(), "PRIORITY=%d", syslog_level(msg.level),
+                "SYSLOG_IDENTIFIER=%.*s", static_cast<int>(msg.logger_name.size()), msg.logger_name.data(),
                 "CODE_FILE=%s", msg.source.filename, "CODE_LINE=%d", msg.source.line, "CODE_FUNC=%s", msg.source.funcname, nullptr);
         }