bitcoin/script.c: Add scriptpubkey_opreturn_padded, which creates an OP_RETURN with a pointless random 20-byte padding.

In the case of `donateutxo`, this is needed since a simple spend of a P2WPKH to an `OP_RETURN` would be below the minimum transaction size.
Sizes below 20 are not plausible as commitments.

bitcoin/script.c

@@ -9,6 +9,7 @@
 #include <ccan/endian/endian.h>
 #include <ccan/mem/mem.h>
 #include <common/utils.h>
+#include <sodium/randombytes.h>
 
 /* Some standard ops */
 #define OP_0		0x00
@@ -221,6 +222,16 @@ u8 *scriptpubkey_opreturn(const tal_t *ctx)
 	add_op(&script, OP_RETURN);
 	return script;
 }
+u8 *scriptpubkey_opreturn_padded(const tal_t *ctx)
+{
+	u8 *script = tal_arr(ctx, u8, 0);
+	u8 random[20];
+	randombytes_buf(random, sizeof(random));
+
+	add_op(&script, OP_RETURN);
+	script_push_bytes(&script, random, sizeof(random));
+	return script;
+}
 
 /* Create an input script which spends p2pkh */
 u8 *bitcoin_redeem_p2pkh(const tal_t *ctx, const struct pubkey *pubkey,

bitcoin/script.h

@@ -30,6 +30,11 @@ u8 *scriptpubkey_p2pkh(const tal_t *ctx, const struct bitcoin_address *addr);
 
 /* Create a prunable output script */
 u8 *scriptpubkey_opreturn(const tal_t *ctx);
+/* Create a prunable output script with 20 random bytes.
+ * This is needed since a spend from a p2wpkh to an `OP_RETURN` without
+ * any other outputs would result in a transaction smaller than the
+ * minimum size.  */
+u8 *scriptpubkey_opreturn_padded(const tal_t *ctx);
 
 /* Create an input script which spends p2pkh */
 u8 *bitcoin_redeem_p2pkh(const tal_t *ctx, const struct pubkey *pubkey,