Revert "REFACTORING: supporting flag about receiving an offset or not."

This reverts commit 05de78e644b1db4a3b7af74f014cf27fd54cea0d.
IO.binwrite doesn't truncate the file if an offset is given
Switch to using ModeFlags in order to specify CREAT and RDWR at the same time
Merge pull request #726 from jruby/io_binwrite_dont_truncate_with_offset

IO.binwrite doesn't truncate the file if an offset is given