This caused a segfault because we have no checkpoints. Added a NULL check
which jnewbery suggested on the original PR, but for some reason didn't
get in.

This commit adds signet support, which is a little bit silly/redundant for us :)
Was a surprisingly easy merge to handle, and hopefully in future Core is more
mindful of signed blocks when they are changing code architecture.

I had to change a couple lines of src/signet.cpp to add blank assets to the
CTxOuts and to find transaction input scriptWitnesses. No need to add any
other tx witness data (and the CAsset()s that I did add to make things compile
won't be used..) because signet will always have g_con_elementsmode off.

More RPC cleanups. This one discovered that the `createpsbt` and
`rawblindrawtransaction` RPCs had incorrect argument lists :)

We have a couple RPC hacks in src/wallet/rpcwallet.cpp that were a bit annoying
to adapt, but nothing too bad.

Found a misspelling of `ignoreblindfail` in the `sendtoaddress` args, and a
discrepancy in the `createrawpegin`/`createpegin` args

Adds "verbose" flag to a bunch of RPC that pass through to SendMoney. I also
added it to sendtomainchain and destroyamount.

I had to make up an onion service target port for Liquid and for our custom
params, so I chose 37041 and 37040 to roughly match how signet just adds
30000 to its normal port.

Spent a while figuring out where this new function should live now that
the PSBT stuff has been pulled into src/psbt.cpp and the wallet, and isn't
mixed up with the RPC logic.

In the end I modified the the function to return a normal error rather
than throwing an RPC exception and put it in src/psbt.cpp with the other
functions that behave this way.

Also fixed an intermittent failure in feature_assetsdir.py that has
been bugging me for the entire rebase.

Changes API for AcceptToMemoryPool to no longer check maximum fee; we remove
this check from interfaces::CChain::testPeginClaimAcceptance. This is fine,
the check never should have been there (this method's job is to notice double
claim attempts and provide a more helpful message to the user; other mempool
failures should be caught later in the `claimrawpegin` logic when we call
`pwallet->commitTransaction` which actually subits to the mempool). It was
only checking fees because the API made it easier to do so than to not.

Update secp256k1-zkp to ff4714e6417c56f7235b287d4a9e555df10e09af latest master,
which is up to date with upstream libsecp 9e5939d2848df5508e3a6eb038273b28d255248c
(pull request ElementsProject#835 Nov 4 2020)

Taproot :D