Better handle groups w/o members
    
    There was a chance that groups w/o members could end up causing a failure to
    store the group. This would happen in case the structure used by glibc to fill
    up the group data was "dirty". Always memset structures before passing them to
    te libc and also check if there are any members, before calling the async
    function.
    Finally add some tracing at level 7 so that it is easier to follow what is going
    on in case of touble.