Fix travel moving player back and forth infinitely

Fix travel moving player back and forth infinitely

This fixes a bug where hundreds of turns are wasted by the travel system
moving the player back and forth when the player targets an unreachable
space and sight-blocking obstacles occur in certain formations
in-between.  The player will only be stopped if they're interrupted
externally, e.g. growing hungry or being hit by a monster.  See the
comment in the code for full details.

Based on DynaHack commit 02da53e (Fix travel moving player back and
forth repeatedly) by me.