log: fix deadlock when using systemd logging - fixes #8621

In this commit the logging system was re-worked

dfa4d94827 fs: Remove github.com/sirupsen/logrus and replace with log/slog

Unfortunately the systemd logging was still using the plain log
package and this caused a deadlock as it was recursively calling the
logging package.

The fix was to use the dedicated systemd journal logging routines in
the process removing a TODO!

fs/log/systemd_unix.go

@@ -5,10 +5,7 @@
 package log
 
 import (
-	"fmt"
-	"log"
 	"log/slog"
-	"strconv"
 
 	"github.com/coreos/go-systemd/v22/journal"
 	"github.com/rclone/rclone/fs"
@@ -18,10 +15,8 @@ import (
 func startSystemdLog(handler *OutputHandler) bool {
 	handler.clearFormatFlags(logFormatDate | logFormatTime | logFormatMicroseconds | logFormatUTC | logFormatLongFile | logFormatShortFile | logFormatPid)
 	handler.setFormatFlags(logFormatNoLevel)
-	// TODO: Use the native journal.Print approach rather than a custom implementation
 	handler.SetOutput(func(level slog.Level, text string) {
-		text = fmt.Sprintf("<%s>%-6s: %s", systemdLogPrefix(level), level, text)
-		_ = log.Output(4, text)
+		_ = journal.Print(slogLevelToSystemdPriority(level), "%-6s: %s\n", level, text)
 	})
 	return true
 }
@@ -37,12 +32,12 @@ var slogLevelToSystemdPrefix = map[slog.Level]journal.Priority{
 	slog.LevelDebug:       journal.PriDebug,
 }
 
-func systemdLogPrefix(l slog.Level) string {
+func slogLevelToSystemdPriority(l slog.Level) journal.Priority {
 	prio, ok := slogLevelToSystemdPrefix[l]
 	if !ok {
-		return ""
+		return journal.PriInfo
 	}
-	return strconv.Itoa(int(prio))
+	return prio
 }
 
 func isJournalStream() bool {