pettycoin-gateway: refund transactions which are too large.

The pettycoin protocol has a limit of ~5BTC, and gateways probably don't
want to deal with anything like that amount anyway.  It's set here to the
theoretical max on the test network, and 0.002 BTC on the non-test net.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
tools/gateway-report.sh: simple inetd serving of gateway logs.

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