Preventing lists from having no privileged owner

[SansPseudoFix]

Version

6.2.60

Expected behavior

Keeping at least one privileged owner per list. Maybe a message when last privileged owner wants to leave the list to force him to choose another privileged one.

Actual behavior

In a list with 2 owners, for example (one privileged and one "regular"), if privileged one leave the list, it remains one regular owner. Users have to ask listmaster to fix it.