Cygwin: newgrp: only allow group from supplementary group list

Cygwin: newgrp: only allow group from supplementary group list

Windows only allows to set the primary group to a group already
present in the TOKEN_GROUP list.  Cygwin OTOH fakes success at
setgid() time, to allow a subsequent call to setuid() to do
the actual account switching.  To have a sane behaviour in the
command line tool, check group membership and disallow to switch
to groups other than those already present in the user token.

Fixes: 8bd56ec873453 ("Cygwin: newgrp: first full version")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>