B15005 narrow openings and hurtling

B15005 narrow openings and hurtling

Narrow openings are currently not checked for by hurtle_step() or
mhurtle_step. This has the consequence that you can jump or use
Newton's 3rd to get through somewhere you can't get through on the
ground, and monsters can stagger or be jousted through somewhere
they wouldn't attempt under their own steam.

This patch fixes hurtle_step().  It does not address mhurtle_step.