Two fixes to the previous patch:

- There was always a bug with which-function-mode where if you enabled
  it when there were already existing buffers, it would behave
  differently than if you enabled it and then created those buffers.
  Namely it would enable which-func-mode for every buffer, even if the
  buffer didn't support imenu.  This is especially noticeable when
  toggling which-function-mode and using the header line, so I fixed it.

- Also, I accidentally dropped a paren before submitting.  Now that's
  fixed :)