Clear planned stair creation in player_place, fixing #1802
Remove A_BRIGHT attribute for inverted colors.

This fixes #1813. As usual, PowerWyrm's diagnosis was
correct. This patch documents the issue and restructures
the code a little bit to make it easier to see what's
going on.
Remove compiler warning.
Refactor target_set_interactive() to have common code for both 'interesting grids' and 'free' modes