seq_file support for /proc/net/atm/pvc:
- pvc_info(): seq_printf/seq_putc replaces sprintf;
- atm_pvc_info() removal;
- the vc helpers (vcc__seq_xxx) do the remaining work.

Chas's suggestions applied since last version:
- atm_pvc_xxx renamed to pvc_xxx
- atm_seq_pvc_fops renamed to pvc_seq_fops