dataprovider: add support for user status and expiration

an user can now be disabled or expired.

If you are using an SQL database as dataprovider please remember to
execute the sql update script inside "sql" folder.

Fixes #57

sftpd/server.go

@@ -260,6 +260,7 @@ func (c Configuration) AcceptInboundConnection(conn net.Conn, config *ssh.Server
 	}
 	connection.Log(logger.LevelInfo, logSender, "User id: %d, logged in with: %#v, username: %#v, home_dir: %#v remote addr: %#v",
 		user.ID, loginType, user.Username, user.HomeDir, remoteAddr.String())
+	dataprovider.UpdateLastLogin(dataProvider, user)
 
 	go ssh.DiscardRequests(reqs)