Add support for the more secure mpm-itk

I informed about the support for different workers like mpm-itk in the freenode #aegir channel, mostly because I consider mpm-prefork to be too insecure, especially in situations where aegir could be deployed.

Ergonlogic suggested creating a support-request to get some information concerning mpm-itk-support and whether, in case there is no support yet, to ask to consider adding it.