Make some trade simulation functions re-usable

[ghubstan]

Code duplication needs to be reduced as new scripts are added.

• Refactor the createpaymentacct functions.
  The steps required to create a payment account are still displayed, but only for Alice, not Bob.

• Treat the trade protocol simulation as an atomic function.
  This will reduce 'main' simulation script size as new ones are added.

• Move createoffer command generation to its own function.

This PRs branch was created off 01-fix-bash-syntax, and #5116 needs to be reviewed and merged before this one.

[sqrrm]

utACK

A bit odd indeed with the atomic function to perform the full trade, but it does what it should in this case.

[ghubstan]

utACK

A bit odd indeed with the atomic function to perform the full trade, but it does what it should in this case.

I wanted the calling scripts to be simpler.
I broke various protocol steps into their own functions -- waitfortradedepositpublished, waitfortradedepositconfirmed, waitfortradepaymentsent, etc. -- in the next PR. That made executetrade a little smaller, but still atomic.