make optional the keeping of whitespace before point on save

[wildlava]

This adds a new variable that enables/disables the current feature that keeps whitespace before point on save, but the default is "t" (true), so default behavior remains the same as before. A user can set it to "nil" to disable the feature.

[lewang]

I added the your customization as a defcustom. The fix for space preservation turned out to be a little more involved.

[wildlava]

One comment: in my patch, I put the (set-buffer-modified-p nil) inside the conditional, since there is no need to alter this flag if the whitespace is not saved in the buffer. The end effect is the same, but I like the idea of the flag not being tweaked if not necessary.