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>

fileserver/utils/dup3.go

@@ -0,0 +1,11 @@
+//go:build linux && arm64
+
+package utils
+
+import (
+	"syscall"
+)
+
+func Dup(from, to int) error {
+	return syscall.Dup3(from, to, 0)
+}