Refactor dwarf wandering for clarity.

Woods' original Fortran condition is

	DSEEN(I)=(DSEEN(I).AND.LOC.GE.15)
	1	.OR.(DLOC(I).EQ.LOC.OR.ODLOC(I).EQ.LOC)

Knuth preserved that hairy condition as a one-liner. But I think
it's much easier to comprehend if we split it up into two separate
ideas: If the dwarf sees you, he begins following you. If you
retreat to the surface, he stops following you. (And otherwise he
keeps doing what he's doing.)

No intended functional change.
Start work on Luckett & Pike's ADV440.
ADV440: FLY, RIDE, and SCORE.
ADV440: implement DROP and READ.
working on adv440
Adv440: dwarf toting, tides.