PROTOCOL_PKT_PEERS support, and peer_cache cleanup.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
netaddr: fix subtle bug in get_fd_addr.

Interestingly, getsockname doesn't *fail* if len is too short.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
PROTOCOL_PKT_GET_PEERS: get peers from network.

We currently only do this when we first connect, but we should probably
be more regular.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>