Tweaks/fixes related to custom fields on upgrade

1. $modSettings['displayFields'] was being set differently than expected, which caused problems (fixes #1887)
2. Tweaked code to set field_order for custom fields (no need to set show_mlist since that defaults to 0 anyway and no need to pull data for all the new fields that already have a field_order)

Signed-off-by: Michael Eshom <oldiesmann@oldiesmann.us>