Move fillPSBT inout argument before output-only arguments. This is a nice thing to do to keep the interface style consistent. But motivation is to work around a current limitation of the libmultiprocess code generator (which figures out order of inout parameters by looking at input list, but more ideally would use the output list).
This PR is part of the process separation project. The commit was first part of larger PR #10102.
Move fillPSBT input-output argument before output-only arguments. This is a
temporary workaround which can go away with improvements to libmultiprocess
code generator. Currently code generator figures out order of input-output
parameters by looking at input list, but it would make more sense for it to
take order from output list, so input-only parameters still have to be first
but there is more flexibility for the other parameters.
