Merge branch 'stable-3.1'

interface/lib/app.inc.php

interface/lib/classes/remote.d/admin.inc.php

interface/lib/classes/remote.d/client.inc.php

interface/lib/classes/remote.d/dns.inc.php

remoting_client/API-docs/client_get_by_groupid.html

remoting_client/API-docs/dns_slave_delete.html

remoting_client/API-docs/dns_slave_get.html

remoting_client/API-docs/sys_datalog_get.html

remoting_client/API-docs/sys_datalog_get_by_tstamp.html

server/plugins-available/mysql_clientdb_plugin.inc.php

server/plugins-available/postfix_server_plugin.inc.php

server/plugins-available/powerdns_plugin.inc.php