slirp: Generalizing and neutralizing ARP code

Basically, this patch replaces "arp" by "resolution" every time "arp"
means "mac resolution" and not specifically ARP.

This prepares for IPv6 support.

Signed-off-by: Guillaume Subiron <maethor@subiron.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>