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.

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

Fixed #4057 Remote API: some API calls are not working because the form definition files are missing