trade screen should use appraisal skill of the unit trading, not that of the "broker"

[ab9rf]

in determining how accurate to display values, the screen should use the appraisal skill of whoever is conducting the trade rather than defaulting to the broker

this gives more reasonable results when the "anyone can trade" option has been selected

[myk002]

Doesn't what we do match what is displayed in the vanilla UI, though?

[ab9rf]

i honestly don't know. the accuracy display is in the viewscreen code, not the caravan handling code, and i don't remember if we even looked into that. we definitely figured out how to determine the raw value, but the accuracy display i don't remember how we decided which unit to use, nor do i recall what the game itself does