This updates generic HDLC from 1.14 to 1.15.

 - fix a kernel panic caused by a recent change to unregister_netdevice()
   (struct net_device * can't be kfreed before rtnl_unlock())
 - adds carrier_* support - hw drivers report DCD status and higher level
   protocols use that info, and do netif_carrier_{on,off}() according to
   DCD and (Cisco and FR) link management status.
 - moves Frame-Relay constants etc. from include/linux/hdlc.h to hdlc_fr.c.
   They are internal FR things and are not needed in the global header.
 - protocol hooks are slighty changed to allow zeroing (memset).
 - removes CONFIG_HDLC_DEBUG_* variables. Users tend to make very wrong
   use of them. Now setting them requires changing .c #define. Anyway they
   are development-only things.
 - misc style corrections etc.