Only use Setsid on Unix.

Setsid is not a part of syscall.SysProcAttr on Windows, so we only set that on
systems that have it.

backend/sftp/sftp_unix.go

@@ -0,0 +1,12 @@
+// +build !windows
+
+package sftp
+
+import (
+	"syscall"
+)
+
+func init() {
+	// ignore signals sent to the parent (e.g. SIGINT)
+	sysProcAttr = syscall.SysProcAttr{Setsid: true}
+}