mISDN: Add E-Channel logging features

New prim PH_DATA_E_IND.

 - all E-ch frames are indicated by recv_Echannel(), which pushes E-Channel
   frames into dch's rqueue
 - if dchannel is opened with channel nr 0, no E-Channel logging
   is requested
 - if dchannel is opened with channel nr 1, E-Channel logging
   is requested. if layer1 does not support that, -EINVAL
   in return is appropriate

Signed-off-by: Martin Bachem <m.bachem@gmx.de>
Signed-off-by: Karsten Keil <kkeil@suse.de>