* io.c (rb_io_seek_m): Accept :CUR, :END, :SET as "whence" argument.
  (interpret_seek_whence): New function.
  [ruby-dev:45818] [Feature #6643]



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40084 b2dd03c8-39d4-4d8f-98ff-823fe69b080e