Fix build for linux_arm64 (#29)

linux_arm64 platform doesn't have syscall.Dup2
so use the nearly identical syscall.Dup3 instead.

daemon_unix.go

@@ -178,7 +178,7 @@ func (d *Context) child() (err error) {
 		d.pidFile.Remove()
 		return
 	}
-	if err = syscall.Dup2(3, 0); err != nil {
+	if err = syscallDup(3, 0); err != nil {
 		d.pidFile.Remove()
 		return
 	}

syscall_dup.go

@@ -0,0 +1,12 @@
+// +build !linux !arm64
+// +build !windows
+
+package daemon
+
+import (
+	"syscall"
+)
+
+func syscallDup(oldfd int, newfd int) (err error) {
+	return syscall.Dup2(oldfd, newfd)
+}

syscall_dup_arm64.go

@@ -0,0 +1,11 @@
+// +build linux,arm64
+
+package daemon
+
+import "syscall"
+
+func syscallDup(oldfd int, newfd int) (err error) {
+	// linux_arm64 platform doesn't have syscall.Dup2
+	// so use the nearly identical syscall.Dup3 instead.
+	return syscall.Dup3(oldfd, newfd, 0)
+}