mISDN: add support for group membership check

mISDN: add support for group membership check

This patch adds a module parameter to allow a group access to the mISDN
devices.  Otherwise, unpriviledged users on systems with ISDN hardware
have the ability to dial out, potentially causing expensive bills.

Based on a different implementation by Patrick Koppen.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Jeff Mahoney <jeffm@suse.com>
Cc: Patrick Koppen <isdn4linux@koppen.de>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>