mISDN: Added layer-1-hold feature

Add IMHOLD_L1 ioctl.
The feature will be disabled on closing.

Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>