* configure.in: Add components/help/converters/* makefiles.
* libnautilus/nautilus-directory.c: Check for NULL directory
  result before doing stuff with it.
* src/ntl-index-panel.c: Ditto.
* src/ntl-prefs.c: Add help browser nav tree to list of possibilities.
* src/ntl-window-msgs.c: Handle non-VFS-handleable URIs correctly WRT 'up' button.