Fix delete-backward-char in haskell-indentation (fixes #211)

The problem in #211 is that while delete-char should only delete the
region if delete-selection-mode is enabled, delete-backward-char
deletes if the region is active and delete-active-region is enabled.
Its default value is t.
Don't show shm's display quarantines in presentation mode
Add a function to guess the module name of the current buffer