Fixed #4425 Variable is initialized as string but later written to as array, throws fatal error in PHP 7.1

Improved the code that generates the ssh keys for clients and shell users and reorganized it into a central function.

i just added a proposed code on howtoforge
https://www.howtoforge.com/community/threads/create-delete-secondary-dns-with-remote-api.74136/

Merged language files.

Datalogstatus notification implementation (as discussed: https://www.howtoforge.com/community/threads/datalogstatus.73719/)