1) As much as I dislike it, upping the collision margin from 0.1 to 0.2
fixes bugs, particularly involving walking into upwards-slanted walls.
2) There were still some problems involving acute crevices/seams; they
were using the adjusted instead of unadjusted normal, and also they need
to bypass the don't-slide-upwards check to prevent (see #6379)
3) The move-away-from-what-we-just-hit code needs to run always, not
just on non-initial iterations. No idea why I did it this way before.
4) Force bullet to give actor boxes a tiny collision margin of 0.001
instead of the default 0.04. I can't tell whether this is actually
working or not, but it should reduce unexplained weirdness.
5) A piece of code that was meant to prevent bugs by short-circuiting
the movement solver if its direction changed more than 180 degrees
actually caused problems instead of preventing them, so I deleted it.
This is the value used by vanilla Morrowind engine. At this angle player
character starts sliding down the slope.
This fixes navigation and movement issue in Mournhold, Godsreach.