psbt: finalize to-remote with option_anchor_outputs.

Until it gains miniscript support, wally doesn't know how to
finalize P2WSH.  This happens with `option_anchor_outputs` for
to-remote, which requires a 1 block delay to force use of
anchor outputs for CPFP.

So we finalize this ourselves.

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

bitcoin/script.h

@@ -149,6 +149,9 @@ bool is_p2wpkh(const u8 *script, struct bitcoin_address *addr);
 /* Is this one of the four above script types? */
 bool is_known_scripttype(const u8 *script);
 
+/* Is this an anchor witness script? */
+bool is_anchor_witness_script(const u8 *script, size_t script_len);
+
 /* Are these two scripts equal? */
 bool scripteq(const u8 *s1, const u8 *s2);