PSBT signing was changed to use SIGHASH_DEFAULT by default in #22514.
The signet miner script sets the sighash type of the created PSBT to
SIGHASH_ALL, hence this leads to a sighash mismatch when the
`walletprocesspsbt` RPC is called. Fix this by explicitly passing the
correct sighash type.
Note that the same change was needed in one of our functional tests,
see commit d3992669df.
Reported by gruve-p.
When mining the first block of a new signet chain, pick a timestamp for
the first block so that after mining 100 blocks the timestamp will be
back to the current time -- this prevents an unnecessary delay before
any miner rewards have matured enough to be spent. This takes into
account that the delta between blocks may be shorter than 10 minutes due
to attempting to increase the difficulty to match --nbits, but does not
take into account the time spent actually generating the 100 blocks.
