psbt: always use non witness serialization format

[guggero]

Fixes #1841.

BIP-0174 states that the transaction must be in the old serialization
format (without witnesses).

https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki#specification (see first entry in table).

cc @Roasbeef, @positiveblue

Fixes lightningnetwork/lnd#6386

[coveralls]

Pull Request Test Coverage Report for Build 2161078427

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 5 of 6 (83.33%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.004%) to 54.135%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
btcutil/psbt/psbt.go	5	6	83.33%

Files with Coverage Reduction	New Missed Lines	%
btcutil/psbt/psbt.go	1	71.59%

Totals
Change from base Build 2148764512:	0.004%
Covered Lines:	24795
Relevant Lines:	45802

---

💛 - Coveralls

[guggero]

This requires a new tag (btcutil/psbt/v1.1.3) after merge.

[Roasbeef]

Should we continue to try to decode the witness version and fall back to non-witness? Since it's possible there's old software out there encoding wit the witness version, we'd potentially break compatibility across lnd version for even btcutil versions with this change as is.

It seems the most important thing here is for us to encode using the new witness version, AFAICT. So if we just serialize w/o the witness, then things are compatible across the prior versions.

[guggero]

I can add the fallback. Though I don't think this change will create a compatibility issue. A PSBT packet with an unsigned TX that has a witness set cannot be serialized, the library will complain. And if you call msgTx.Serialize() with a transaction that has no witness, the old format will be used. Therefore the change to the serialization is purely to be more explicit about it. The actual important change is that we always read the TX expecting it to be in the non-witness format.

[Roasbeef]

LGTM 🛰

[Roasbeef]

The tag btcutil/psbt/v1.1.3 has been pushed.