prevent crazy point adjustment

Under a seemingly harmless condition, set `disable-point-adjustment' to
t using `post-command-hook' to prevent the event loop's usually helpful
point adjustment from going berserk.  The following was also added as a
comment:

  So we ended up at the beginning of a section, which also is
  the end of the preceding sibling section, if any.  There are
  two markers here.  If the preceding section is hidden, then
  we are also at the end of an invisible text range.  So far
  so good.

  But if we ended up here because we moved upward (but not when
  moving downward), then "point adjustment" messes up badly.
  Visually point would still be in the right spot but according
  to the function `point' we would be somewhere else; at the
  beginning of the preceding section, which isn't even the
  first position "at the other end of the invisible text".
  The existence of markers appears relevant, but not their
  insertion types.  Wtf!

  If we are not at the start of a section then do not disable
  "point adjustment", because when we truely end up *inside*
  an invisible range, then moving outside is a good thing.