Missing call to setgroups before setuid

[jayvdb]

Repeat of jeroen/sys#24

Also

missing-call-to-chdir-with-chroot: This executable appears to call chroot without using chdir to change the current directory. This is likely an error and permits an attacker to break out of the chroot by using fchdir. While that's not always a security issue, this has to be checked.

If these functions are going to be kept in unix, it would be a good idea to provide wrappers for these functions recommended to be used with the existing functions.