Various fixes for bugs and cross-platform issues.

- Use canonical paths just about everywhere in the help browser. We want
  to stick to portable, normalized paths as much as possible, to avoid
  funky Windows path names, especially in bookmark files which should be
  portable across different platforms.

- Fix the generated click_toc() call in display_toc(); we need to
  properly stringify the callback argument there.

- Update the search field after opening a directory via the file
  browser, so that the search text matches the directory display. Is
  there a reason that we're not already doing this? I don't think so,
  because we're doing the same when clicking a link in the toc.

- Prevent the doc directory from being bookmarked. There's nothing
  interesting to see there anyway (just subdirectories, no help
  patches). Also, since the toc lives there (in a virtual sense, i.e.,
  the current browser directory is set to this directory whenever the
  toc is displayed), we also avoid an interesting race condition which
  arises if we try to update the toc while we're displaying it. (This
  doesn't seem to happen on all platforms, but I've seen it on the Mac
  at least.)

- Fix a purely cosmetic issue with the bookmark entries. (Entries with a
  null description were stored as is and then written to the bookmarks
  file. Now the description is just omitted in that case.)