txprepare: fix all amount when other amounts also specified.

This is legal!  And we actually do this in tests, but we didn't check the psbt
was spendable (the next patch does, indirectly, by testing feerate):

```
        # Discard prep4 and get all funds again
        l1.rpc.txdiscard(prep5['txid'])
        # You can have one which is all, but not two.
        prep5 = l1.rpc.txprepare([{addr: Millisatoshi(amount * 3 * 1000)},
                                  {addr: 'all'}])
        # Feerate should be ~ as we asked for
>       assert normal_feerate_perkw - 1 < feerate_from_psbt(bitcoind, l1, prep5['psbt']) < normal_feerate_perkw + 1
E       AssertionError: assert (7500 - 1) < -1091803.9574935874
```

Changelog-Fixed: JSON-RPC: `txprepare` with `all` as well as one or more non-all amount fields now produces a valid PSBT.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

plugins/txprepare.c

@@ -285,6 +285,20 @@ static struct command_result *psbt_created(struct command *cmd,
 
 	/* If we have an "all" output, we now know its value ("excess_msat") */
 	if (txp->all_output_idx != -1) {
+		/* Subtract any other outputs they specified! */
+		for (size_t i = 0; i < tal_count(txp->outputs); i++) {
+			if (i == txp->all_output_idx)
+				continue;
+			if (!amount_sat_sub(&excess, excess, txp->outputs[i].amount)) {
+				return command_fail(cmd, FUND_CANNOT_AFFORD,
+						    "Could not afford output %zu", i);
+			}
+		}
+		if (amount_sat_less(excess, chainparams->dust_limit)) {
+			return command_fail(cmd, FUND_CANNOT_AFFORD,
+					    "Could not afford 'all' output: only %s left",
+					    fmt_amount_sat(tmpctx, excess));
+		}
 		txp->outputs[txp->all_output_idx].amount = excess;
 	}