server: clean supplementary groups when setuid

[luizluca]

When nbd-server drops privileges, it was leaving supplementary groups untouched. As nbd-server was normally dropping from root, nbd-server kept membership to root supplementary groups.

Signed-off-by: Luiz Angelo Daros de Luca luizluca@gmail.com

For example:

$ cat config
[generic]
    user = nobody
    group = nobody
    oldstyle = false
[export1]
        exportname = /dev/zero

$ sudo ./nbd-server -d --config-file config.example

$ ps -eo pid,comm,euser,egroup,supgrp | grep nbd
 5068 nbd-server      nobody   nogroup  root,socked

After the patch:

$ ps -eo pid,comm,euser,egroup,supgrp | grep nbd
 4958 nbd-server      nobody   nogroup  (null)