* fixed special bits for maildrop and public dir
After changing the group, special bits are lost, but they should be set for the directories `/var/spool/postfix/{maildrop,public}`, otherwise you see the following error:
```
postfix/postdrop[17400]: warning: mail_queue_enter: create file maildrop/729504.17400: Permission denied
```
* fix: Match octal permissions originally provided
Officially Postfix source seems to imply:
- `730` for `maildrop/` (_but has mentioned a sticky bit in the past, set-gid bit only for the postdrop binary involved_)
- `710` for `public/`
Both folders are assigned the same group that `postdrop` belongs to which has the SGID permission for it's executable. SGID special bit on`public/` doesn't seem necessary, but left as-is to match the default from Debian.
---------
Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>
