make timestamp-file respect the 80 column limit

Turn the lambda into a proper named function and format the resulting
code properly.
make automatic timestamp update configurable

Add new configuraiton variable 'fvwm-enable-last-update-timestamp'
used to enable/disable adding the timestamp-file hook.
remove call to kill-all-local-variables

This call was interfering with the correct working of
define-derived-mode, which already calls kill-all-local-variables
itself.
treat single quotes as whitespace