(tpu-before-save-hook): Rename from tpu-write-file-hook.

(tpu-before-save-hook): Rename from tpu-write-file-hook.
Activate it with add-hook on buffer-save-hook.
(newline, newline-and-indent, do-auto-fill): Use advice instead of
redefining the function.
(tpu-set-scroll-margins): Activate the pieces of advice.