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

This removes logrus which is not developed any more and replaces it
with the new log/slog from the Go standard library.

It implements its own slog Handler which is backwards compatible with
all of rclone's previous logging modes.

fs/log/syslog_unix.go

@@ -5,7 +5,7 @@
 package log
 
 import (
-	"log"
+	"log/slog"
 	"log/syslog"
 	"os"
 	"path"
@@ -39,7 +39,7 @@ var (
 )
 
 // Starts syslog
-func startSysLog() bool {
+func startSysLog(handler *OutputHandler) bool {
 	facility, ok := syslogFacilityMap[Opt.SyslogFacility]
 	if !ok {
 		fs.Fatalf(nil, "Unknown syslog facility %q - man syslog for list", Opt.SyslogFacility)
@@ -49,27 +49,27 @@ func startSysLog() bool {
 	if err != nil {
 		fs.Fatalf(nil, "Failed to start syslog: %v", err)
 	}
-	log.SetFlags(0)
-	log.SetOutput(w)
-	fs.LogOutput = func(level fs.LogLevel, text string) {
+	handler.clearFormatFlags(logFormatDate | logFormatTime | logFormatMicroseconds | logFormatUTC | logFormatLongFile | logFormatShortFile | logFormatPid)
+	handler.setFormatFlags(logFormatNoLevel)
+	handler.SetOutput(func(level slog.Level, text string) {
 		switch level {
-		case fs.LogLevelEmergency:
+		case fs.SlogLevelEmergency:
 			_ = w.Emerg(text)
-		case fs.LogLevelAlert:
+		case fs.SlogLevelAlert:
 			_ = w.Alert(text)
-		case fs.LogLevelCritical:
+		case fs.SlogLevelCritical:
 			_ = w.Crit(text)
-		case fs.LogLevelError:
+		case slog.LevelError:
 			_ = w.Err(text)
-		case fs.LogLevelWarning:
+		case slog.LevelWarn:
 			_ = w.Warning(text)
-		case fs.LogLevelNotice:
+		case fs.SlogLevelNotice:
 			_ = w.Notice(text)
-		case fs.LogLevelInfo:
+		case slog.LevelInfo:
 			_ = w.Info(text)
-		case fs.LogLevelDebug:
+		case slog.LevelDebug:
 			_ = w.Debug(text)
 		}
-	}
+	})
 	return true
 }