{nl,cfg,mac}80211: add a parameter to enable mcast retries

For some mesh applications we may want to transmit multicast
frames multiple times.  The RMC cache will discard duplicates
based on the sequence number in the mesh header.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
{cfg,nl,mac}80211: add a separate mcast ttl

This ttl value allows adjusting the ttl of mcast frames separately
from unicast mesh frames.

Signed-off-by: Bob Copeland <me@bobcopeland.com>