[minor] autoexplore:confuse on fully explored levels(>3 out)

[notallwhowander]

Autoexplore is generally good about going between the entrance and exit of a level after it's fully explored. However, when there are more than two ways out (such as up, down, and a temporal rift in daikara), it only goes between two of them. This is worst with the old battlefield entrance - autoexplore gets stuck going between two adjacent graves after returning to the surface, once the level is fully explored.