MaxScale does not drop supplementary groups if --user is used

rpmlint warns of the following:

missing-call-to-setgroups-before-setuid:

This executable is calling setuid and setgid without setgroups or initgroups.

This means it didn't relinquish all groups, and this would be a potential

security issue.

This means that if the user who starts MaxScale is a part of some supplementary group and the user given with --user is not, the lack of a initgroups() call means that the user ends up being able to act as if it was a part of this group.