seq_file support for /proc/net/atm/arp:
- svc_addr/atmarp_info(): seq_printf/seq_putc replace sprintf and friends;
- arp_getidx/arp_vcc_walk() take care of the usual seq_file cursor
  positionning: they both return NULL until the cursor has reached its
  position. struct atm_arp_state is updated accordingly;
- arp_seq_{stop/start} are responsible for clip_tbl_hook (un)locking;
- module refcounting is done in arp_seq_open()/arp_seq_release();
- atm_lec_info() is removed.

Chas's suggestions applied since last version:
- atm_arp_xxx renamed to arp_xxx;
- atm_seq_arp_fops renamed to arp_seq_fops.

Chas didn't ask for it but I renamed arp_vc_walk to arp_vcc_walk.