Reduce dselect memory usage

Reduce dselect memory usage

* dselect/baselist.cc (baselist::startdisplay): Create the
pad with the list of the size of the display and not of the
size of the list content itself.
* dselect/basetop.cc (baselist::refreshlist): The part to
display is always at the top of the pad.
(baselist::redrawitemsrange): Simplified to redraw the real
range only.
* dselect/pkglist.cc (packagelist::sortmakeheads): No need to reallocate
the pad when the list changes.
* dselect/pkgtop.cc (packagelist::redraw1itemsel): The line
of the item in the infopad doesn't correspond to its index in
the list any more. Adjust accordingly.