postfix: add setgid wrapper for postqueue/postdrop

Both postqueue[1] and postdrop[2] implement a subset of administration
task that are supposed to be run unprivileged users
and require the setgid bit to full-fill this task.

[1] http://www.postfix.org/postqueue.1.html
[2] http://www.postfix.org/postdrop.1.html