memory provider: load users from a dump file

The `memory` provider can load users from a dump obtained using the
`dumpdata` REST API. This dump file can be configured using the
dataprovider `name` configuration key. It will be loaded at startup
and can be reloaded on demand using a `SIGHUP` on Unix based systems
and a `paramchange` request to the running service on Windows.

Fixes #66

service/sighup_unix.go

@@ -0,0 +1,23 @@
+// +build !windows
+
+package service
+
+import (
+	"os"
+	"os/signal"
+	"syscall"
+
+	"github.com/drakkan/sftpgo/dataprovider"
+	"github.com/drakkan/sftpgo/logger"
+)
+
+func registerSigHup() {
+	sig := make(chan os.Signal, 1)
+	signal.Notify(sig, syscall.SIGHUP)
+	go func() {
+		for range sig {
+			logger.Debug(logSender, "", "Received reload request")
+			dataprovider.ReloadConfig()
+		}
+	}()
+}