seq_file support for /proc/net/atm/lec:
- lec_info(): seq_printf/seq_putc replaces sprintf;
- traversal of the lec structure needs to walk:
  -> the lec interfaces
     -> the tables of arp tables(lec_arp_tables);
        -> the arp tables themselves
     -> the misc tables (lec_arp_empty_ones/lec_no_forward/mcast_fwds)

  Sum up of the call tree:
  atm_lec_seq_start()/atm_lec_seq_next()
  -> atm_lec_get_idx()
     -> atm_lec_itf_walk() (responsible for dev_lec/dev_put handling)
        -> atm_lec_priv_walk() (responsible for lec_priv locking)
           -> atm_lec_arp_walk()
              -> atm_lec_tbl_walk()
           -> atm_lec_misc_walk()
              -> atm_lec_tbl_walk()

  Each of the dedicated functions follows the same convention: return NULL
  as long as the seq_file cursor hasn't been digested (i.e. until < 0).
  Locking is only done when an entry (i.e. a lec_arp_table) is referenced.
  atm_lec_seq_stop()/atm_lec_itf_walk()/atm_lec_priv_walk() are responsible
  for getting this point right.
- module refcounting is done in atm_lec_seq_open()/atm_lec_seq_release();
- atm_lec_info() is removed.

Chas's suggestions applied since last version:
- atm_seq_lec_fops renamed to lec_seq_fops;
- change in state handling: it wasn't correctly set to its reset value
  after a complete interface walk;
- lec_arp_get_status_string() bugfix.