add suid group and only allow execution when in suid group

[hilbix]

Rise the security bar a bit further: Only allow to execute, if user is in suid group.

• For this we need to add a suid-group, of course, and remove other execute permission.
• Also add some self-check of the permission, to make sure, everything is correctly set up.
• Note that the group membership of the program then changes (but does not need to be checked itself)
  which means, that we perhaps must switch to the correct group first.

[hilbix]

This should be part of #4