[x11/slock] Does not unlock - Set-group-ID on exec and setting group to 'auth' fixes it.

Hi,

x11/slock stopped working for me -- after locking the screen with
`slock` I would not be able to unlock the screen with the correct
password.

I was able to fix this issue with:

# chmod 2755 slock
# chown :auth slock

Reporting it here so that it can be fixed before the 6.7 release.

ps:
I'm running openbsd snapshots.

- rsiddharth