Helpers for seq_file conversion of proc/atm/{pvc/svc/vc}:
- struct vcc_state keeps
  1) the struct sock from which the current struct atm_vcc is deduced
  2) the family to which must belong the vcc (PF_ATM{SVC/PVC/any})
  3) the availability of clip module
- vcc_seq{start/stop} are responsible for vcc_sklist locking
- __vcc_seq_open and vcc_seq_release take care of get/put for the clip module.

Chas's suggestions applied since last version:
- atm_vc_xxx renamed to vcc_xxx
- atm_vc_common_seq_open renamed  __vcc_seq_open (future name clashes
  avoidance)