sh: Fix some bugs with exit status from case containing ;&.

sh: Fix some bugs with exit status from case containing ;&.

Also, rework evalcase() to not evaluate any tree. Instead, return the
NCLISTFALLTHRU node and handle it in evaltree().

Fixed bugs:

* If a ;& list with non-zero exit status is followed by an empty ;; or final
  list, the exit status of the case command should be equal to the exit
  status of the ;& list, not 0.

* An empty ;& case should not reset $?.