ipg: run-time configurable jumbo frame support

Make jumbo frame support configurable via ifconfig mtu option as suggested by
Stephen Hemminger.

Cc: Stephen Hemminger <stephen.hemminger@vyatta.com>
Tested-by: Andrew Savchenko <Bircoph@list.ru>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>