Various pickup problems

[lahm86]

Some pickup/item issues in V1.8.1:

• Similar to enemy drops, one-item limit should set the unobtainable_pickup count in the gameflow to avoid confusion when checking the statistics.
• When skidoo drivers are converted to pickups (to reduce savegame usage) sometimes the items are unreachable despite being floored.
• If randomizing secret rewards, but not secrets in TR1, extra reward items are placed in non-existent reward rooms.
• A particular secret in Bartoli's Hideout can become unreachable after blowing up the final area.
• If Dragon's Lair has an additional door before the arena, pickup items on the neighbouring tiles (which become walls) are no longer reachable.