Fix ptrdiff_t limits introduced in r24654 (!)

* The ptrdiff_t limits are PTRDIFF_MIN and PTRDIFF_MAX, not PTDIFF_*.
* I could not find any non-Haiku reference to PTDIFF_*, so I guess
that's a mistake.
Style fixesStyle fixes.
BScrollbar: remove unused field.

* Nothing ever reads fTargetName in the scrollbar code, so remove the
field.
* Frees one reserved slot, and a little memory, as the target name was
copied with strdup.