Ticket #3000: incorrect file size is reported in copy/move overwrite query dialog.

(overwrite_query_dialog): adjust file size.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
size_trunc_len() generates incorrect result in some cases.

Revert "(size_trunc_len): support automatic maximum unit for specified size."
This reverts commit ef94b84eaf570dd1cc3faa383cdb49a7638c3be6.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
(size_trunc_len): minor optimization.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>