Add flag to disable timestamps for process log (#2781)

* Add flag to enable/disable timstamps for process log.

solves #2615

* Remove timestamp argument from log.Printf()

* Add log-timestamps=false to systemd unit

* Copy log flags

* Fix argument list

caddyhttp/httpserver/logger.go

@@ -171,7 +171,7 @@ selectwriter:
 		}
 	}
 
-	l.Logger = log.New(l.writer, "", 0)
+	l.Logger = log.New(l.writer, "", log.Flags())
 
 	return nil