Issue #809, restructure position init code.

Issue #809, restructure position init code.

Modify startup sequence, so position can be modified late in the startup process, right before the scenery load starts. This allows two ugly hacks to move to a permanent, less hacky location. If other position init modes required similar late evaluation in the future, this can be accommodated now.

This is a somewhat high-risk change - I've tested both carrier starts and runway-selection based on realwx METAR, but please look out for other position-init issues and test before / after this patch.