Update commentary.

Update commentary.
(dirtrack-debug): Doc fix.
(dirtrack-mode, dirtrack-debug-mode): New names for
dirtrack-toggle and dirtrack-debug-toggle.  Use define-minor-mode.
(dirtrack-toggle, dirtrack-debug-toggle, dirtrackp, dirtrack-debug):
Make obsolete.
(dirtrack-debug-message): Only print message if
dirtrack-debug-mode is non-nil.  Use with-current-buffer.
(dirtrack): Doc fix.  Use dirtrack-mode rather than dirtrackp.
Remove dirtrack-debug checks now that dirtrack-debug-message does this.