Redirect STDERR to error log (#709)

* Redirect STDERR to error log

* Add dup2.go

* Redirect error log to log file

---------

Co-authored-by: 杨赫然 <heran.yang@seafile.com>

notification-server/dup2.go

@@ -0,0 +1,11 @@
+//go:build !(linux && arm64)
+
+package main
+
+import (
+	"syscall"
+)
+
+func Dup(from, to int) error {
+	return syscall.Dup2(from, to)
+}