Add TX_TO_GATEWAY.

This is just like a normal tx, except:
1) The destination must be a valid gateway.
2) The outputs are lost to the pettycoin network.

We rename gateway to from_gateway in various places for extra clarity.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Check gateway address is known gateway for TX_TO_GATEWAY, add test.

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