dusty-htlcs: enforce limit on dusty htlcs

for every new added htlc, check that adding it won't go over our 'dust
budget' (which assumes a slightly higher than current feerate, as this
prevents sudden feerate changes from overshooting our dust budget)

note that if the feerate changes surpass the limits we've set, we
immediately fail the channel.

channeld/commit_tx.c

@@ -35,6 +35,22 @@ size_t commit_tx_num_untrimmed(const struct htlc **htlcs,
 	return n;
 }
 
+bool commit_tx_amount_trimmed(const struct htlc **htlcs,
+			      u32 feerate_per_kw,
+			      struct amount_sat dust_limit,
+			      bool option_anchor_outputs,
+			      enum side side,
+			      struct amount_msat *amt)
+{
+	for (size_t i = 0; i < tal_count(htlcs); i++) {
+		if (trim(htlcs[i], feerate_per_kw, dust_limit,
+			 option_anchor_outputs, side))
+			if (!amount_msat_add(amt, *amt, htlcs[i]->amount))
+				return false;
+	}
+	return true;
+}
+
 static void add_offered_htlc_out(struct bitcoin_tx *tx, size_t n,
 				 const struct htlc *htlc,
 				 const struct keyset *keyset,