net: cap inbound connections per address

A single IP could hold every inbound slot up to MAX_CONNECTIONS -- which
is what a connection-exhaustion flood did to the rendezvous relays, one
address parking hundreds of sockets and starving real peers. Add
MAX_CONNECTIONS_PER_IP (32): the accept path counts existing connections
from the same address and refuses a new one past the cap. The relays'
firewall connlimit does the same a layer lower; this makes every build
carry the defense, including future poll()-based relays with no FD ceiling.

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