Modify the behavior when the messages TBM_SETSEL, TBM_SETSELSTART, and
TBM_SETSELEND are sent and TBS_ENABLESELRANGE is not set.  When the
style TBS_ENABLESELRANGE is not set, Windows observed behavior is to
set the Selection Start and End values to 0, rather than leave them
unchanged.

This is needed, because symbol addresses in dwarf2 debug info are already relocated.

dbghelp: Fix up load_offset to work in case the load offset is different for real binary and debug link file.

Make sidx signed so sidx < 0 comparisons work.
(most of the patch written by Eric Pouech)